Lots of questions young hobbit!

I'll answer the first one: yes you can stir up a lot of trouble with spies. I use groups of 10 mixed with 5 or 6 Bishops/Alims to really get the ball rolling. It is a very effective tactic, especially when you use it against a vastly superior foe that you are at present allied with. I had three of these destabilizing groups wandering around France while the Almohads and Egyptians tried to settle who was boss, and they both wound up having to abandon Europe. Not bad! Pull them out immediately, though, if you get a faction uprising or they will foment a rebellion against them, probably with the guys you were trying to kick out in the first place.