EmperorFool
Deity
I replaced the CvGameCoreDLL.dll file with the copy from the "Backup before Dresden" folder, and reinstalled the HOF mod. I can start a new game from the Custom Game menu when I am running the HOF mod (Lock Modified Assets was already checked as a default), but I get a "not valid for HOF use" warning.
The Unofficial Patch installs some XML and Python files as well to the Assets subfolders. In each subfolder where it does this, it has a backup_before_dresden_0.21 folder to contain the originals. You'd need to replace all the originals over the new ones and delete those backup folders.
It seems like the best solution might be to uninstall and reinstall BtS using the official patches, then install BUG and HOF and try it again.
Yes, this is the surefire solution.