Well, it went from bad to worse after I installed the most recent Windows Updates (as I was getting ready to follow your instructions). Now I guess something's wrong with the display
driver, because it boots up to just a black screen now, and now whenever I try to reset or shut down from Safe Mode it just hangs at that Shutting Down screen eternally.
So, I think it's time to call it a day. I've backed up important files to a USB drive and am going to just format and re-install Windows from scratch sometime tomorrow. Case closed, I guess?
Thanks for taking the time to try.