Just tried ACW CORE v2.1, and I must say, the Invasion Mode is great!
However, a few problems and suggestions:
1) Death Knight's Death Coil are useless on Demon Pit Lord, and the Level 8 Doom Guard. Why don't just change their classification into non-undead?
2) I think you should add two cinematics for the ending of Invasion Mode.
Victory -- celebration, all demons die and the Devil yells angrily about his defeat
Defeat -- Devil conquers ACW world, demons overrunning it and all heroes and 'mortal' bases die
3) I suggest that you should import useful custom Abilities, that sync with each others' effects and make the abilities more useful.
4) Or make a seperate map for Invasion Mode, where the bases are owned by one AI player, allowing 11 players to join the fun. Perhaps name this map as ACW Extreme Invasion?