@Corka,

I just made a new post about building Coalitions, but so far that only works when you have:

1. Reliable Status
2. Supreme Power Status
3. An alliance with the Papal States
4. The Papal States are Supremely Powerful (two Supremely power states allied together makes for a tough combo, plus the Pope won't backstab you)
5. Use a "Crusade" to fight against a common enemy, preferably one of a different Religion (although I did get the Milanese out of the game pretty quickly after they got excommunicated for attacking me)

Here's the post I just put up:
https://forums.totalwar.org/vb/showthread.php?t=87879

Hope it helps. It's made for a VERY fun Middle Eastern 6 (me as Venice, Hungary, Denmark, Papal States, Egypt [yeah, they wanted in on it too], Poland) on 2 (Mongols and their strong Byzantine allies) skirmish so far!