I'm a bit confused as to how you can have a variable difficulty level across human player civilizations in an MP game. It seems impossible, for example, that the computer players could somehow benefit only from the Noble-level difficulty settings when targeting the Hyborem player, versus Immortal when targeting the others, since so far as I know the difficulty setting does not affect combat odds, except against barbarians. Even more unfathomable is the bonuses AI gets: does this stay at an immortal level difficulty for everyone?
In the end I'm guessing you just mean the penalties levied against players on higher difficulties, such as health and happiness caps? That is moot as far as Hyborem is concerned, and almost as much with Basium since he shares the summoning civ's resources. Are there any further penalties, perhaps additional unit costs/lower free unit cap, that might have a real effect on gameplay?