Across the seas, across the waves.

At a minimum, preventing the AI from fleeing battles is a matter of a single script line.

And aren't the starting AI heroes already scripted to prevent them from doing so? They never ran away in my games, and I was sure that was intentional.

Maybe add shackles?

Undoubtedly. Although, I didn't really need them. I prefer to explore everything, so while my Necromancer slowly (checking every nook and cranny) and relentlessly went to defeat one enemy, a Sanitarium with 8 Scouts prevented the Elves from building up an army behind him, and a second portal exit was guarded by a Deleb with a handful of Phoenixes. But the AI was really acting strangely; the Dwarf didn't leave his little area, the Elves were probably fighting the Humans, and only the Mage attacked me at the beginning. I think it's just a matter of luck here. As for the difficulty, I think it's fine – there's always a point where the enemies don't matter anymore, but I did struggle with the Mage at the beginning. Yes, it would be nice to somehow deal with the distances using one-way portals to a conditional center (transport hub) of the map.