In my Aragon game, I was the HRE, with prestige 100, infamy 0, high trust with everyone in the game (I was HRE, Papal controller and everyone thought I was highly trustworthy, to be trusted implicitly/utterly) with royal marriages with everyone I could get them with, and still it was Austria and England which were forming PUs and inheriting countries outright - not by war or claiming thrones, but by sheer luck. About the only thing that may not have been great for me was my ruler's diplomatic skill - I had a 4 diplomacy ruler. So - if I got an ambassador advisor, would it help me form personal unions through royal marriages (without using claim thrones/fabricated documents)? What finally determines who inherits a kingdom when a ruler dies without a heir? Is it just chance?