The first two issues are not the norm. I haven't seen any major reports of those issues, nor has it affected my 4 windows 10 machines or my friend's computers.

Because it had been uninstalled. By windows. Because it felt like it.

I seriously doubt this happened. The 1151 update uninstalled a handful of programs due to them causing issues, but blender wasn't one of them. A google search doesn't come up with anything regarding Windows 10 automatically uninstalling blender.

And that doesn't help much, because it bogs my PC down just as much.

https://puu.sh/nrV9T/0de5c323ef.png Literally every program below that white line uses less system resources than defender.

A modern system shouldn't be noticeably affected by an AV sitting in the background.

(Common Sense is easily the best AV program there is, been running Windows 7 with no Antivirus OR updates for the past 4 years with no problem, the Windows 10 virus excluded)

I don't really care about your lack of AV, since if you get infected it's not my problem (and if you did, you probably wouldn't even notice anyways). However running a OS without updates for 4 years is just irresponsible use of a computer and isn't in the realm of "common sense" at all. It's not something to boast about, it's just dumb. It might even be why you get issues when performing an upgrade.

I could be dead wrong, but I have a feeling that I'm not

lol, Microsoft isn't silly enough to alienate the userbase they've been building by charging a monthly fee for windows. That'd be commercial suicide at this point.

Frankly, it sounds like your windows 7 install isn't in great shape and the issues are just made worse when you upgrade to 10. If programs are magically disappearing, it might even be a registry issue.

This seems more like a case of PICNIC - Problem In Chair, Not In Computer.