@DoctorOhh -- thank you very much! I did the configuration folder option that you suggested above -- next best thing to a fresh install.
But it pretty much confirmed my suspicions of something goofy in the settings. I've used the portable since v. 0.8.17 and done each update -- habit, thanks to Adobe and Microsoft.

Something just went awry this time.
The portable actually serves as sort of a backup (aside from being portable), since I also run the desktop version and it's doing just fine. So everything is back to normal.
As far as a single directory and all the special restrictions, that's just way more thought than I want to invest right now. I use Calibre for storage, transfer-to-device, and converting the occasional file to a different format. Perhaps I should petition Kovid for a program for us "simple" users.
Thanks for your help. Your expertise is greatly appreciated.