if you cannot blow it out, you can buy a mirror cleaning tool and cleaner to wipe it off. check with your camera store, or if you do not have a camera specialist store in your area, call b&h photo and they will be happy to advise you. you can get their number on the internet.

suggest you invest in a gimbal mount for your tripod. nice lens

be advised that jpeg images may lose quality over time, raw will not. that being said, be careful with free programs, research completely, as some of them are imbedded with viruses, malware, and irratating ads.
find an inexpensive program like photoshop elements or lightroom and you will have a program that you may use for years with great results.

photoshop is a post processing program designed to enhance your photos. if you are not shooting raw photos, then you probably do not need photoshop, as there is a huge learning challange to using it. you may enjoy Lightroom more, as it is easier to use and will allow you to enhance your photos for a better presentation.
as to the cost, photoshop is available with a monthly fee only. unless you are able to pirate an older copy, you cannot buy the program outright. the program is offered by adobe and can be researched (or googled) on the internet. there are many other programs that you may buy outright to edit your photos. check with your local camera store or major electronics store and ask questions.

in my opinion, you should shoot in whatever mode yields the photo quality you are looking for. when i am shooting a landscape, i will shoot manually to get the best photos i can, i have time to adjust and experiment. when i am shooting wildlife, i choose aperture mode and let my camera make the necessary adjustments, there is no time to adjust when shooting flying birds, they will not stay and pose. when i am at a family or friends gathering, i may choose to go fully automatic so that i can capture pics and still have fun with the party. that is why we spend so much money to buy the technology to travel so many avenues. shoot the way you enjoy shooting, learn along the way, and you will enjoy photography for the rest of your life. struggle with learning and trying to capture photos the way you think the pros think you should and you will fight your natural instincts and will be done with photography in 6 months to a year.

go to cabelas or bass pro and get a bean bag gun rest

when you shoot an ff camera in dx mode, you lose pixels. on the nikon d750 you would go from 24 megapixels to about 18 megapixels. you would be better off to crop the photo in post processing. when you shoot in crop mode, you are not actually getting a larger image, it is just cropping the photo on the camera sensor instead of cropping the photo in software. the only time you would need to use the dx mode is if you are having a hard time positioning the image while taking a photo. a dx setting on an fx camera with a 300mm lens does not physically make the 300mm a 450mm lens. it just seems like it does.

i shoot with nikon, so i am only versed to that brand. that said, i would recommend a nikon d7100. a less expensive version would be a nikon d5300. both are great cameras, i just think the d7100 is easier to learn where all the controls are. you can buy either used on bhphoto.com or adorama.com for a reasonable price.
if you do not think you will ever get serious about photography in general, then just use your cell phone. they all take great pics for selling items online.

try red river paper online

lightroom, you can choose just the face you need to adjust

i shoot with the nikon d750. it is a great camera, that said, any of the three you listed are great cameras. but for the price difference, if you choose the nikon d610, you may be better off changing to the d750. check the reviews online.

if budget is a concern, try someplace like b&hphoto online and buy used or refurbished. i have purchased several lenses from bhphoto, they rate the condition of the lens, and if you stay with 8.5 to 9 or higher, i have never had a problem.

whatever you get, make sure it is an fx lens, not dx. that way when you decide to upgrade your camera to a full frame camera like a d750, you will be ready to go. that said, the 35mm lens or the 50mm lens will be your best choice.

upgrade to a d7100 or d7200. will work with all your lenses and will add desirable features to what you have already learned how to use

don't judge all because of one idiot. you asked a good question and received some good answers. some people just like to be mean, ignore them.