I think the inheriting lands thing works only 1) when your current queen is from that faction, 2) you have really good happiness across your kingdom. Not sure on whether allied/neutral makes a difference, but I'm sure being at war prevents it.

Allies can't be trusted (except, funnily enough, the Vikings in VI - once you get them onside they tend to honour the alliance through to the bitter end - ie when you attack them ) A number of times I've had the situation where an AI faction has sent me a princess one year, and an invasion the next! The lesson is don't drop your guard just because you have an alliance. It's not as pronounced an effect as it is in STW though, where you can be pretty much sure that if an enemy offers peace and you accept, they WILL attack again the next year. It seems like accepting a peace offer is seen as a sign of weakness. This effect is still there in MTW but watered down a lot and modified by other things, I'm sure.

Likewise, I tend to use princesses for keeping generals a bit more loyal, that is their main value to me, to be honest. (I even dropped one on a prince last night to head off a civil war, and of course he died the next year, and his "successor" general still had the "secret incest" vice, even though he was no longer married.... at least the loyalty boost carried across to him )