Scripts

1) How to change a hero's specialization

A straightforward solution is to place the hero on the map, click on "shared" in the properties tree, and there will be many hero parameters that can be changed, including specialization. As far as I remember, this will turn your map into a mod. You can also create your own hero and customize it as you like; this solution will not affect other maps.

2) How to remove the player turn bar

If the computer does not have any active heroes or castles, you can remove the turn bar using the function with the self-explanatory name DoNotGiveTurnToPlayerAIIfNoTownsAndActiveHeroes().

There is a quest to defeat the demons.

You can give the demons names (and if they are heroes, they already have names by default, but there are some specific features for heroes), and check for their presence on the map using the IsObjectExists() function based on some event. The most convenient event is probably the battle result trigger:

Trigger(COMBAT_RESULTS_TRIGGER, 'CombatResult');

function CombatResult(id)
  if the quest is active and both demons do not exist, update the quest
end

Also, don't forget about non-standard options, for example, check when issuing the quest whether the player has already killed them.