1. If you don't get them, soon they'll get you
2. Yes, sort of. 1.5 left some hardcoded diplomacy stuff in that 1.01 didn't. Not sure why... You have a better chance of just trading and avoiding war though with most factions if you are smart with your diplomats. Evil factions don't have access to diplomats right now
I can change that if requested... Just plan on evil vs good and vice versa. Evil factions can team up for sure though, don't dismiss this option.
3. To be specific, in 1.5 the starting regions of the main Dwarf kingdom is just smaller. There are a number of rebel dwarven kingdoms that aren't specifically Chaos, just independent of the current rulership of the Dwarven King! Also Hashut engineering corps is sexy to an artilleryphile, hence so many converts.
4. MANY differences! I suggest you set up a single player custom battle and add all 4 chaos factions, then go into the units and browse away. The main differences here are Bodyguard units, lesser and greater demons. You'll see patterns that are somewhat like this: Khorne is high charge, Tzeentch is high attack, Nurgle is high HP, and Slaanesh is high defense (agility).