Tried another test game as Dal, and got a bit further. I think I'm starting to get the hang of this. I kind of want to do an AAR as Dal once I actually get to the point of playing it for real...
One thing I've learned is that - and I'm sure this is no surprise to anyone here, but it's something I had to learn - you can play the same character in the same place with the same strategy, and the game will still go differently, because NPCs' personality traits are all semi-randomized. This time, for example, Prince Erik refused my attempts to betroth him to my daughter even after I killed his wife for him. I did, though, pick up two alliances with fellow pagan counts, including the lone holdout in the south, which kept him in the pagan camp this time around. And this time I managed to forge a claim on a neighboring county and, with the aid of my allies, mop the floor with 'em. But I still couldn't get anyone to join the Erik-for-king faction, even Erik himself. At this point it's almost looking like it might be easier to just do it militarily: grab as much land as I can, make as many alliances as I can, conquer a duchy, and then launch a civil war to make my own damned self queen. Screw Erik. (Literally, in the unlikely event he'll agree to a matrilineal marriage.)
Anyway, at that point I went back to my desperation tactic of trying to assassinate the existing heir in hopes that if I just murder enough people, Erik will be the only potential heir left. Also spying on the king and trying to smear him with rumors. That didn't work, and I got caught, but before it became a problem, I managed to forge a second claim. I was winning, too, but I forgot to never lead my armies personally, got an infected wound, and died... Leaving my Christian chancellor in place as regent for my heir. So, going to try again.
I'd appreciate some input: how actually useful is the diplomacy attribute for my character? I've been setting her up as a Diplomacy+Intrigue machine, with Elusive Shadow and Attractive, but I'm wondering if it might make more sense to focus on Martial+Intrigue instead, given how most of my success so far has been through underhanded dealings and just straight-up marching armies over people. On the other hand, alliances have been absolutely vital to what successes I've had, but I'm not sure how big an impact my diplomacy stat had on that, and how much was being Attractive. And at this point, there's an appeal to just stomping on people's faces with boots.