It sounds like you've done the testing to prove it is indeed file related - specific files cause the problem.

Now that you've isolated things down to specific files - wow, I have never heard of that. I don't use Word myself (I use LibreOffice), but I'm sure Word has some "document protect" function whereby you can prevent editing the document ("lock it", in other words). However, the problem you describe does not sound like that at all. The fact that you can re-save the file to a new name and the problem goes away is what made me think of the "file protect" thing. But some of the stuff that's failing for you is not file editing. e.g., "Ctrl + A = Select All". Sure, you generally do that before performing an edit, but making the selection itself is not editing per se.

Hmmm, ... you have a weird problem, that's for sure!

You might try a different keyboard if you have one handy, just to rule that out. Often times you cannot isolate what's causing a problem, but you can start eliminating the things that could cause it, and hopefully whittle the list of suspects down to where you can infer the cause, by process of elimination. Especially if things are intermittent - you can reproduce them most of the time but not 100% of the time - you can easily get tripped up trying to declare a cause.