Bishops are just priests with the "bishop" trait, they arent a completely different agent (although they get their own model).

just like cardinals are just priests with the cardinal trait.

I actually send my priests in before I take the provence, a few priests, spy and assassin.

Spy invades city, and the assassin hits the church first and then any +order buildings, priests move in and convert the populace to christendom.

That usually gets the city fairly "unrestful" and a fair few times has caused a full on riot kicking out the muslim owners, at that point I can move in and "save" the populace from their islamic tyrants. Leave a couple of priests to bring it up to the 90%+ christianity bracket while everyone else moves on.

Do it right and I dont even have to go to war with the faction i'm invading!