Re: The importance of both politics and diplomacy in a campaign
I do similar things, these strategies work work in my experience. I like having an ally, so there's a border I don't need to defend, and I like the way that for military allies, their regions count as ours for the victory conditions (so we don't need to betray an ally to win). Of course, having an ally can help with getting trade agreements with other factions, as they tend to like you if you're friends with nations they like.
How much do you try to expand your family tree? I normally keep trying to expand it, through marriage and adoption and by sending children to be tutored (to give them a better chance of reaching adulthood and so I'll have good candidates for the next generation of commanders). However, I tend to expand my ruling family too much - this gets expensive in both coins and the loyalty of rival parties.