#1: When a Lady is killed in a House this is taken as an offense to her married House as well as her birth House.
#2: Marrying a Lady from an adversary is supposed to be a way to bury the hatchet and broker peace.
#3: Lands are passed down first through male heirs and their sons (except Dorne and the Iron Islands.) If there are no more sons of the oldest male heir, then the lands go to next oldest brother and his sons. If there are no more male heirs in that family then the lands go to the oldest female heir. (Tywin relayed to Olenna that if Loras doesn't produce an heir, then when he dies The Reach lands go to Margaery.)
As of S4E2 the line of succession over who is king next at King's Landing is discussed here:
As of S5E3, it has been noted that if a marriage between a couple is not consummated then the Law of the Land is that it's not really a marriage (this was told by Little Finger to Roose Bolton.)
It is unclear if this true, however Lord Bolton acknowledges this by stating he believes Little Finger when he says Sansa is still a virgin.
Game of Thrones: Threads at the TV Critic