Raptor,
Know exactly how you feel since I also have scoliosis and can't handle the weight/DSLR. I personally use a Canon EOS M with zooms and they are very light and work great for me. For you I would recommend selling your existing setup and going with a Micro 4/3 because it is much lighter than your current setup and has the 2X multiplier factor on the lenses. So my pick would be the Olympus OM-D Mk 10 II which is on sale now for about $499 and the Panasonic 100-400mm (effective 200-800). The camera with battery and memory card weighs a dash over 13 ounces and the lens weighs a tad over a pound. That means your entire setup weighs around 30-33 ounces, which is more than 50% less than your current rig. Your back will love it and I think you'll be happy with the results. PS. All Micro 4/3 lenses fit all the cameras since they have a standard mount.

Being retired you need something light and simple that produces excellent photos. There are lots of options, but I would pick between a Nikon or Canon digital SLR. Both are excellent. I'm a Nikon man so I would recommend the Nikon D3300 with the 18-55mm and 55-200mm lens. This entire package is being sold by B&H for about $540 (This price includes a $300 rebate from Nikon so this is a great price). This package will take care of 99% of the photo opportunities you will run into in your retirement travels. You may want to add a 70-300mm lens when you go on safari for the extra range. Some will recommend a longer lens in the 400mm+ bracket, but you may not want to deal with the extra weight. Some may recommend one lens instead of 2/3 lenses such as the 18-200mm or 18-300mm, but I have always found these lenses a bit of a quality compromise for the convenience of 1 lens. I would recommend you go to a camera store or a store like Best Buy and feel the Nikon and Canon equivalent and see which one feels best in your hand. Nikon has always felt better to me in my hand, but you'll have to decide.

https://www.bhphotovideo.com/c/product/1119766-REG/nikon_13473_d3300_dslr_camera_with.html

There is no doubt that the camera world changed with the cell phone. The reality of todays world is that things can change very quickly regardless of how much we may have liked the old product, system etc. I never imagined that digital would ever replace film, but it did. The reality of the print world is that 99.9% of all prints are no bigger that 4 x 6. So most people could care less about 16 x 20 or bigger. When you look at camera sales there is no doubt as to what is happening. Compact camera sales have all but died with the cell phones. Sales of DSLRs and Mirrorless are down. What's the future? Who knows. My guess being a computer guy is that a similar fate could be facing DSLRs/Mirrorless as happened with computers. By that I mean for about 20 years there was this ever growing hunger for more processing power. We went from 64K, 128, 286 etc. Eventually we got to around 2.5Mhz and the market died because people had all the processing power they needed. Well in my opinion the digital market has grown from 1MP to now 24MP is the standard. You can essentially do whatever you need to do with 24MP, which will probably slow significantly future sales. We'll see what happens.

Ronicas,
My wife and I have been to Yellowstone about 10 or so times in our lifetime so we know it pretty well. I did a bit of research before giving this answer so this is pretty close to ground truth. The only entrance to wheeled traffic in the winter as has kinda been mentioned is the north entrance or Gardiner, Mt entrance. This is 233 miles from Cody and would take about 3 hrs and 40 minutes to drive. There will probably be 15-20 ladies in your art class, so you might try to convince someone with a car to possibly take this side trip with you since it is a chance of a lifetime to see Yellowstone in the winter. To my knowledge I could only find snow tours (tracked vehicles) that go into the part from the North (Gardiner), West (West Yellowstone), and South (Jackson). I was unable to find any tours from the east (Cody). Why? My guess is that as someone mentioned Cody does shut down in the winter, and it is 50 miles just to get to the park. With a track vehicle that means it would probably take 2 hours just to get to the park one way from Cody. Since Cody doesn't have an airport you must be flying into either Bozeman/Billings, or maybe Jackson airports so you are being driven a fair distance. If you really want to see Yellowstone it will take a plan and some effort because it is pretty remote, and the weather can be very tough in the winter. I remember going through the park years ago and it shocked me that I couldn't pick up 1 radio station on the AM dial. Good luck and have fun.

I'm a retired military man and we have a saying about "knowing your enemy". My wife and I have been to yellowstone many times, and I would say you're 55-250mm, which effectively goes out to 375mm because you have a cropped sensor, will handle probably handle 90% of the photos. I say that because you will be piddling along in your car around the figure 8 road that goes through the park. Game is not really afraid of the public so they are pretty close to the road. You pull over to the side of the road and will have some pretty close shots of elk and buffalo herds. There will be some more distant shots, but you can walk into the fields to get closer. In regards to lenses under $1,000 you have basically the Sigma 100-400mm and the Tamron 150-600mm. The only problem is that the Sigma weighs about 42oz and the Tamron 72oz. Now your Canon 55-250mm only weighs around 15oz so the difference is huge. If you do elect to get the lens I would recommend the following technique. You will be driving slowly through the park and see some animals either fairly close or farther away. These animals will typically be eating or standing, but in no particular hurry. I would keep the big lens handy for a quick change if necessary. Otherwise I would use the light canon lens for most of the shots. Another alternative would be a Kendo 1.4x teleconverter. It runs about $160 at B&H and has very good reviews. This would take your canon out to about 500mm. I would probably try the teleconverter because of weight. If nothing else you could buy it and try it before you go to yellowstone.

I went to Israel in 1984 and for me I had a 35-85mm lens and I thought it was perfect. Everyone has there favorite lens/lenses. Your 28-300mm could certainly do a great job, but you have to decide if you want to carry a 44oz lens 8 hours a day while your touring and getting in and out of the bus. I'm assuming you will be on a tour so you will be guided everywhere except for I think 1 free day. You will be in cities probably 80% of the time (Jerusalem, Bethlehem, Jerico, Nazareth, etc). The other times you will be out in the desert at the Dead Sea, Masada, Sea of Galilee etc. If you want a smaller zoom something like the 24-85mm would be good and it weighs about 20oz. I would definitely have some sort of zoom to give you the flexibility.

Being honest, none of us knows the future. Some facts that are true is that for one compact cameras are virtually dead. This segment has been taken over for sure by cell phones. The pro segment for sports, fashion photography, weddings etc will always be there because of the demands of the business. What is also true is that sales of the DSLR, mirrorless cameras has been on glide slope down for the past 5+ years. What we don't like to admit as consumers is that business is business and they are there to make money. None of us could have predicted 20 years ago that Kodak would have died or the demise of the film industry. I think there will be some survivors, but I think there will be some fatalities. I went to a wedding a year ago and I was the only person with a camera. Everybody else was using a cell phone or tablet. Todays reality.

Let me first say that I went to my wife's 20, 30 and 40th high school reunion and I was the event photographer so I'm very familiar with what you will be going through. You will be taking casual photos and then the formal class picture probably after the dinner. Your 450D will do a good job. I shot my first reunion on film and the 30th on a 6MP camera and the 40th on a 12MP camera. In the case of the last 2 I sent a CD to the organizer of the reunion and my wife's friends to distribute to the class. I would recommend that you get a good flash because with even a small class of 50-80 students you will be back 20 to 30 feet for the class picture so your flash will have to have good range. I wouldn't chance available light because you never now what the lighting will be in the restaurant/hall. This is a once in a lifetime picture, and probably the last reunion you'll go to, so I wouldn't chance it with bad light.
The one caution/variable I'll mention is the size of your class. If it is a larger class in the 150 or bigger category you may have to think how you will shoot the photo. With a smaller class you normally have woman seated in the front row with woman also in the next 1 or 2 rows with the men in the rear. In a larger group you may have to shoot from a step or chair or have the class on steps so everyone is visible. I also agree with the comment on being familiar with the equipment. Years ago I took a different camera to a wedding and all the photos were underexposed. Lesson learned. Good Luck.

Let me first say that as in many things in life you have to consider the entire "package" in regards to any choice. This is particularly true when it comes to digital photography because you have all these variables like sensor size, internal camera processing, external processing, software, printers, and then these other factors like weight, cost etc. Having said that we all choose what is important to us at that point in our life and the requirements we have. I remember a discussion I had with a professional gov't photographer in Washington D.C. about 10 years ago where he said he went to a news conference to take pictures of the President with another photographer in his office. The other had the top of the line Canon full frame and he had an APS-C sensor camera. His buddy couldn't figure out why his prints of the president were so much better than his on the big Canon. My buddy said that the reason was that he used an Epson printer, which at the time was much better than the competition. I have a bad back and have gone mirrorless and am reasonably pleased with the results. It's performance is much better in low light than my DSLR because of newer technology, but overall I must admit that the DSLR takes sharper photos. It's probably a combination of focusing, processing etc. That is my personal opinion from my experience with the cameras I have. You will have to decide what' most important to you. There is an old saying that out of 3 factors when choosing you can pick 2, but never all 3. You can't have it all. If you want a sports car you can't carry 7 people.

You are basically looking at a compact camera that will fit in her purse. I'm a Nikon guy, but I must tell you that if you look at the compact comparative tests over the years the Canon's do best and in particular the Elph's. They just seem to have a knack of putting together a small camera that just does a good job. There are several different models, but the 160 seems to get good reviews and it's not much over $100. Like I said the camera just seems to do this specific job of a small compact very well at a very reasonable price. You can pay more, but you may not get more.

Let me first say that as you can see in your answers so far there are many different recommendations/choices to your question. As mentioned you could choose a super compact like the Sony RX3/4, you could choose one of the many mirrorless choices available or a lighter DSLR. I injured my back and was forced into a mirrorless camera that weighs 20oz total with lens. I prefer looking through a viewfinder, but that's what I had to do. You mention the move to the D7100 or D7200. Your D800/e cameras weigh about 35oz with battery and card. The D7100/7200 weigh about 27/28 oz with battery and card. So in my opinion the 7 to 8 oz difference is not a big enough difference from your D800. I would personally recommend considering the D3300. Why? Because it weight 17oz with battery and card. That's nearly 20oz lighter! It has a similar 24MP to the D7100/7200 and all the essential capabilities. Yes the D7100/7200 have more features, BUT what got you going down this road is WEIGHT. I will also make another comment/recommendation. Lenses - There are specific uses for F2.8 lenses, but in general most photographers don't need them because they normally shoot in sunlight/good light, and bodies today are so good in low light. Herbert Keppler, one of the great photo journalists of all time, had an article 15 years ago about why he shot with variable aperture F3.5-5.6 type lenses vs F2.8 lenses. It was mostly weight, but he actually compared very detailed resolution tests that showed results were pretty close as long as you didn't shoot wide open or closed. So you could save another 20oz by using a lens like the Nikon 18-55MM or 55-200MM, which are light and pretty good. You'll have to decide what works best for you.

There is no perfect answer because different people will chose different tools to do the same job, but I have been to both Israel and have done numerous weddings so I will give you my opinion. I would take the 24-85mm lens that you currently have and leave the rest at home. Why? On the Israel trip you will be on the move on a tour every day, and you will be in towns most of the time. You will fly into Tel Aviv and be taken by bus to stay in Jerusalem. You will stay in Jerusalem for several days seeing the sights so the short zoom will be perfect in town. You will then go to Bethlehem (town), Jericho (town), Dead Sea, Masada, etc. You will be in and out of buses and walking your feet off. The 16 oz 24-85mm is the perfect carry around lens. It is also the perfect lens for weddings in my opinion. Yes you can use some other lenses for weddings, but the short zoom for me works best at weddings. We like to throw money at lenses sometime for fun. I don't think you need to.

I have shot numerous weddings and know exactly what you are talking about because as you say you are shooting pictures for 12 hours. In my opinion no strap or flash bracket will fix the problem, which is that the weight of your system is to much for a 12 hour shoot. Your camera is not the problem because it weighs a reasonable 31 ounces. Your lens/lenses weigh in the vicinity of 40 ozs hanging off the front of the camera. My first suggestion would be to go with a lighter lens. You could go with a lens like the 24-85mm, which weighs around 16 ounces or an older AF zoom like the 35-80mm, which I think weighs around 10 ounces. I have shot numerous weddings with the 35-80mm and it is plenty sharp. You are going to be shooting 90% of your photos with flash so you don't need the 2.8 tank. A second suggestion would be to have an external battery pack for your flash. Most flashes use 4 AA batteries, which have some weight. I personally don't use an external flash because I don't want to fool with the cord, but it does save some weight. You will probably be so happy with the lighter lens that you won't worry about the flash.

I've been shooting photos for over 50 years and I can't figure out the fascination with shooting indoors without flash. If you must then I would recommend the 50mm F1.8 that you can pick up for about $150. The 35MM F1.4 is about $1,000. As the saying goes light is life in the photo business. Yes the new cameras are great in low light, but indoors at night in a house for the most part has crappy light. Somehow some have grabbed onto the fact that no flash is more "natural" which is "always" superior. That is not true. Yes indoor photos with light from a window is nice, but a great flash photo is also great. Remember that probably 90%+ of indoor and outdoor wedding photos(depending on sun) are taken with flash. Why? It's a once in a lifetime event and you can't chance that poor lighting in a church, reception etc gives you marginal pictures. Shooting without flash has it's place, but shooting with flash also has it's place.

Every photographer has his preference. I can tell you that when you're sitting in row 23 you will be looking at nearly 1/4 to 1/2 the court. Not nearly enough. The recommendation on renting a 70-200 was a good one. I used an 85mm F1.8 at a Tae Kwondo tournament and I was in row 3. I found it wasn't nearly long enough, and the fixed focal length of 85mm was really confining because I obviously couldn't zoom either in or out. After a few shots I quickly switched to a zoom. My motto is that the light won't help you if you can't get the shot/photo. That's my opinion, but if you only have those 2 lenses I would take both to see what works best for you.