I've used the Canon S90, S95, S100, S110, S120 and love them all!

Both the lens and the body have switches for autofocus. The body switch is near the lens mount at the bottom on one side.

Dust spots on a sensor are almost always lighter and very round. I shoot across a lake a lot and there are times I just have to give up because of insects. As far as being on a filter or lens, it's virtually impossible for depth of field to be high enough to be sharply focused on the lens or filter. I'd say bugs. I have lots of cameras and shoot a lot so sensor cleaning is a normal process for me. I've seen lots of dust spots on sensors and they bdon't look like the darker elongated things on your picture. (I had to hit the + sign to get the shot larger to see them after the download)

I have 5D lll and 5D4 as well as Nikon D810 cameras. My Canon lenses are usually better quality, mostly L series. I have to use a little unsharp mask in Photoshop to get the Canon shots as sharp as the Nikons. No big deal! There isn't an anti-aliasing filter on the Nikons so they tend to be sharper for that reason only. There are settings in the menus for sharpening if you shoot jpegs. There is a definite difference. I shoot a lot!!!

The old lenses are called breech lock and the later are bayonet type. They are completely interchangeable. I've used both and could not care less which I use.

I use one and have found it to be quite good.

https://kenrockwell.com/nikon/2485afs.htm

I have several and love them, but get extra batteries! Adapting old manual focus lenses on mine has been fun.

One of the first things I do in class is to have students adjust diopter correction on the camera to match their eyes needs. First, just point the camera to the sky or a bright light source and adjust the diopter until the little lines or focus marks on the focusing screen are sharp. That's a rough adjustment. Then point the camera at a contrasty subject like a sign, allow the camera to AF, maintain your distance and see if you can touch up the focus anymore. I've had students exclaim Wow after doing it. Old eyes will often need correction from time to time in different light, so make it a habit to check it often. Set it with whatever glasses you'll be wearing while shooting. Also, on another point, the pentamirror of most APSC entry level cameras is inferior to the pentaprism on better models, and full frame cameras have larger and easier to focus screens. The diopter adjustment has no affect on AF, but rather allows you to accurately see what the AF is doing. Cheers!

I own use both cameras and find the Pentaxes to be more intuitive and natural. It's a little like comparing apples to oranges though. I love experimenting with and adapting older manual lenses on the mirrorless cameras. When I want to do serious work without having to think about the camera I prefer a DSLR. I grew up on SLR cameras so they are more intuitive to me. Others may have the opposite experience. As for megapixel numbers it's overrated to me. Dynamic range has long been the greatest issue since I shoot lots of landscapes. I like using older K mount legacy lenses on the Pentaxes and like the pentaprism viewfinder vs pentamirror on other cheap DSLR cameras. Cameras are just tools and I use the best one for the job. Battery life is also much better on DSLR cameras. After watching my retirement funds on the stock market rollercoaster I decided to cash much of it and rather than a motorhome or expensive motorcycle I started buying/selling/trading cameras. I always buy used and I've done much better than the stock market while having much more fun. I teach photography so knowing the workings and menus of different models helps immensely. I'm just an old guy living life to the max and sharing as much as possible. I tell students to quit worring so much about equipment, and start shooting what you love. Cheers!

When it comes to macro lenses for the most part there are no bad ones. They are easy to design and I'm unaware of a bad one. Your D750 also has the advantage of coupling and focusing with any model AF Nikon lens. I would suggest staying in the 90-105 mm range to stay back a little from the subject. Manual focus lenses are less expensive and in macro-micro distances AF is of little help if your eyes are well tuned to your camera.

If you buy generic hoods thay are pretty cheap, and about the same quality. Until then I use clear fingernail polish on the hood to slightly build up the worn mount. Actually works pretty good sometimes.

Prague was one of my all time best photo experiences. A wide angle lens is a must because of narrow streets around the castle, and inside churches. Cemeteries are really cool. There is the WW ll Terezin concentration camp which is a bus ride away that was incredible. A train ride gets you to Karlstejn Castle which is amazing. The contrasts between old and new, and effects of communist vs democratic governments, and the fact that Prague was almost untouched by WW ll bombing makes it a really interesting place to visit. Public transportation was very good. I was there in '94 not long after The Velvet Revolution and the effect of communism on the people was still evident, or seemed so to me. There seemed to be a lack of trust and openness. People on trains and buses rarely talked to me or each other. English was rarely spoken or understood. Just my experience, your's may be different as citizens have become more adjusted to the new government. I never felt unsafe and carried a backpack of medium format camera gear. The trains and stations reminded me of scenes from Doctor Zhivago, very primitive.

That lens won't autofocus on the D3300.

In 95% of my shooting needs the center spot only is used. I go into the menue to choose that option. Then I place the spot where I want to focus and gently touch the shutter release just enough to let the camera focus, recompose if needed and push the release the rest of the way to fire the camera. Shooting with very shallow DOF with fast lenses, like with portraits, it works fast and accurately. Faster and easier than moving focus points. For moving subjects that are hard to track, like flying birds I use the many focus points. I teach photography and to make a point I had students try both ways, focus-lock and recompose, and moving the focus spot. It was much faster to lock n shoot. I know where I want the focus, the camera usually just looks for contrast. Here are some off center, lock n shoot examples:

Having owned or used almost very brand and kind of digital camera out there the biggest issue to me is battery life. I love my Sony A7 ll but the battery life is so bad I have to carry several batteries and a charger on day trips. My Nikon D810 and Canon 5D 4 are much better, Nikon beating the Canon. I never have to change a Nikon battery during the day, just charging at night. The Nikon has the best overall image quality based my extensive use. I'm sure my comments will light up the internet with defensive views, but that's my honest evaluation. I use them all, and love them all. It's really hard to pick an overall winner.