Not sure if it's a good idea to let a rising star manage an autonomous territory.
Makes me think of the duke of Normandy that conquered England and the 100 years wars.
It's a big stretch to link the Duchy of Normandy that had special rights created in the 10th century and 100 years wars that began in the 13th century but for me it can only means troubles for the next generations.
(That's why I think that a marriage might help, it can also worsen the problem if there is a bad luck involved (if progeny/descendant problems occur), anyway it seems to me like the best idea for now)