Problem taken care of. I still have no idea what the heck was going on though. I decided to spring for Drive Genius and scan the problematic volume from an internal drive I can boot from also just for such tasks, but it found no errors of any kind. None of its other tasks helped either.
I decided to migrate my wife's account into that backup installation of OS X... worked great. Rebooted back to the main drive, deleted her account, recreated it, and re-copied a backup of her entire old Home folder contents into the newly-recreated one. Works fine now. Hopefully it stays fixed.
Thanks for all the input.
