Thanks for taking a look, and glad they were of interest. I think I eventually posted the outcome of that campaign (although without commentary); wound up able to complete the Minor Victory through diplomatic means; got a bunch of military allies to reach the 30-region condition. It was easy to get the allies because I sent an army raiding through Spain, sacking a bunch of WRE towns. This made pretty much the rest of the world fall in love with me as if I were a basset hound puppy.

As for the succession, I don't know for sure, because I don't think I've run into that specific situation. On the face of it, a great question; in history, I'd have to say "yeah, the guy has a great claim". But I don't think territory-acquisition-by-marriage is a mechanic in this game. (It's kind of an interesting idea for future TW titles though, especially if/when there's an Medieval III).

Instead, my impression is that the FT displays the Rugian king mainly just to show that's where your daughter went, and a reminder that you have a diplomatic marriage active with that faction. The game considers your daughter to have joined his family, but not vice versa. Notice that neither one of them have any influence/loyalty values, and do not have political actions available. Just to be sure, try designating him as your heir just to see if he's even eligible...I'm 99.99999% sure that you won't be able to.

I do know that even if it were possible, it could only happen if you have no other Faction Heir designated at all. You can choose pretty much any male member of your family as Heir, doesn't have to be a legitimate or bloodline direct descendant. Can be an uncle, cousin, brother-in-law, nephew, etc. Just be careful about rank, traits, legitimacy/adoption status. A low-rank bastard is likely to result in a pretty big loyalty hit, among both your own family and the Other Nobles.

If your Faction Leader dies without a designated heir, then the game will automatically choose one for you, and will default to a male descendant....even if it's a young boy. If the heir is a minor, then his mother will rule as Regent until he comes of age. This happened to me in my first campaign when I was still trying to figure out the basics of FT management...lol, you definitely want to avoid this, it has a detrimental effect on loyalty, influence, and control.