A possible workaround for this problem; likely one that others have reached, but I haven't seen it listed:
- Using a program such as WinRAR or 7zip, unpack both your most-recent save and one that precedes the occurrence of the area reset bug. Create a backup copy of each for the sake of safety.
- Manually compare the two archives; copy into a separate folder the .fog and .lvl files for any areas that are found in the unbugged file but not the most-recent save, plus those for areas (like the Stronghold Main Hall) which were bugged. It's not difficult to do, as the .fog/.lvl files are numbered.
- Insert those copied files into the .rar of your most-recent save, overwriting the bugged areas and recreating those that got deleted entirely.
I've only played a couple hours with my new, Frankensteined save, but so far everything seems to be working properly. I have regained access to all the areas that were cleared in the earlier un-bugged file, without affecting the progress made between the point where the bug happened and when I noticed it.