More timeline. Feel free to ignore.
As of this date, Merrybell is doing just fine. I have NO IDEA what we did to fix it, either. Feaelin suggested booting it up in Safe Mode, which presumably not only brings OS X online with minimal drivers, but also puts the HDD through some kind of test/repair thing. This seems to have done the trick, as Merrybell has behaved itself ever after, and is passing all of the Disk Utility checks with flying colors.
Interesting.
I am not going to start waving any banners yet, obviously, as there's nothing saying the recent issues aren't just the harbinger of more serious concerns to come. But I appreciate the reprieve on its death sentence. I do love that poor little thing.