Okay, update:
I had nothing better to do today, so I actually did do the whole reinstallation dance a second time. Once I downloaded and installed the whole thing as 1.2B1, sans Collectors' Edition content, I checked and saw that the game worked (as expected). I then made backups of the string files for an experiment, installed the Collectors' Edition content, and then updated the game back up to 1.2B1.
I was going to see if tossing in my copied "functional" string files would make it work, but when I went to do the control test for my little experiment, expecting to see lots of "string not found"... I found the game working fine.
As such, it appears it may be a matter of just reinstalling the game until it starts working right, or something.
I'd provide my "working" string files in the Library, to let someone else see if some goof-up along that line could be the culprit--but seeing as how it's only days until 1.2 beta 2 is out, there's not much point. I did search the screens.str file for any would-be new content in it, and found [TABLEALIAS OptionsModsWnd]; you could open up your file and do a search for that, and know that there's a problem with at least the screens.str file not updating properly (although finding the table's present doesn't mean there can't be some other problem with the file).
