So I just got my first campaign win. As hilarious as it was storming the Sorcerer King's castle was with a doom stack of eleven champions, I'm guessing that it's probably not intended that I be able to ally with all seven rivals at once. There are a number of ways to game the diplomacy between them, since they don't actually get pissed off and join the SK unless you actually talk to them while allied with someone they hate. If you manage to get favor from quests you already have, and through mana donations during trade requests, you can end up allied with them anyway.
On the subject of diplomacy, I'm fairly certain that the Tyrant's Indoctrinate ability doesn't work. At least, learning it didn't seem to impact favor at all.