The GСК (presumably referring to a game mechanic or feature) is problematic. I personally don't use the "underground" feature in GСК at all. Should we fix this as part of the NHF project?

Yes. The issue lies in the modified staircase files. If "y" is set to -2 (as it was) instead of -1 (as it is now), the guards will be created.
0
-2
However, the transitions won't work without a mod. Are these modified files essential, or can we do without them? Also, for many heroes, the skills and abilities don't match the table. For example, Baltazar has strong chaos magic instead of skilled magic. Furthermore, Hiorrmel learns spells strangely. He only learned Regeneration at level 3 of the guild, Teleport at level 5, and decided not to bother with Holy Word and Resurrection at all. I also didn't notice how Sergio gets additional experience.