1) Re Pre-emptive actions: The "uncover plots" action for spymaster has been changed to "scheme". You can set your spymaster on vassals in factions and blackmail them into leaving factions. The probability of this happening is also quite decent.
2) Re Happy Vassals joining factions: You need vassals to be more happy than +49 to stop them. I am not sure what the actual limit is, but they have mentioned this in the DD that deals with faction... if you have super happy vassals then they won't join factions.
3) Re Fast growth: You can appease them by, say lowering crown authority, introducing laws in their favour, giving them patronages etc... earlier, the King was too powerful, now the vassals have also gained power. It is hard for King to be too abusive towards vassals, and it is equally hard for the King to just expand the realm without worrying about the situation with vassals. It adds a new dimension to the game...
4) Re Immunities: I have two words for you: Magna Carta. The baron formed a faction and forced the King of England to sign the thing that limited his authority (and basically stopped England from ever turning into an Absolute Monarchy seen almost everywhere else in Europe at some point or other). Besides, vassals are not breaking any law by just demanding things. They are breaking law by going to war, after which they become traitors...
5) Re old saves: Yeah, the old saves suck for this reason... I'd imagine on newer games, you'd not have that much amount of blobbing in the first place unless the monarch was exceptionally skilled.