Today, I found some time to play again – six months of game time have passed, and the map has reached a stalemate:
In Inferno, Sigrif has amassed more forces than the blue demons, so they are hiding from him in the corners, while he stands in the center near Hatabich and gathers enough troops from the two castles under his control to win against Kobelekh. However, he clearly doesn't have enough strength to defeat Kobelekh yet (he has made several attempts). I still haven't figured out how to reach the undead castle without losses, and it's not worth it anyway – there are five times fewer forces there than I have gathered in the controlled castles.
In Shi'o, the local demons have completely cleared the territory of neutrals, visited all the upgrade locations, and completed all the quests, and now they are simply collecting tribute from the mills (periodically appearing greens are destroyed quickly and without losses). All castles have been upgraded to the necessary and sufficient minimum to supply the maximum number of creatures and resources without unnecessary costs.