Right clicking is right clicking, but if you want to change a file name (even right from the icon on the desktop), it’s two single left clicks. Click on it, then click on it again, not a double click.
Yes.
If you’ve really had those files on your computer since the 90’s (so, either DOS, Win 3.1 or Windows 95/98/ME), the file names probably got corrupted at some point. It’s not uncommon for a file to mysteriously lose it’s extension and end up with something like what you have. Especially if the files ever moved back and forth between a system that only allowed 8 character (plus extension) file names and ones that allowed longer ones. So, if you created a file at work/school on a Windows 95 box and called it "bookproposal.txt, and then stuck the disk in your home computer with Win3.1, it might turn it into bookprop.osa.
Here’s how I screwed up an entire directory of files by attempting to manipulate them in DOS without realizing that DOS couldn’t handle the longer file names. To this day, what, 30 years later, when I still get a little twinge of anxiety every time I see a file that ends with ~1.