I know how to unlock all factions. Though it requires digging into the games files and 'tweeking' it. Byzantines are however, hard. You have all the Catholic Factions against you, as well as the Turks. And man, if you get a crusade on you... Oh boy.
I find that playing England is easiest while HRE is hard or hardest on the beginning factions. I can't recall which is the hardest when all of them are unlocked... Either Byzantine, Spain, or Portugal. I have half a mind to go Portugal, pain in the arse it is.
I finally can play on multiplayer with steam. If anyone wants to play, let me know. Now... time to post and edit my previous ones..