Continuing the exploration for iwd1.
Neither ielister or ieparse detects any save game difference besides the starting gold, but that was an expected possible outcome, since the iwd2 offset would put it in unknown territory. However, comparing the files manually confirms there's no other difference, so for iwd1, GAM is not affected.
Loading a HoF save without nightmare mode on in the ini reverts it back to normal. Meaning the HoF state wasn't saved anywhere else in iwd1.