@Brimstone Absolutely, the possibility of getting an heiress to a clan of assassins/spies/informants to join him, yeah, that's a golden opportunity, he'd be a fool to not at least try.
This entire arc is about securing the princesses support, and I wouldn't burn any bridges, the mercs and ninja clan will only take him so far, he needs proper allies, the princess will probably want to tie him down after the war, which I would accept, but he'll most likely refuse for various reasons.
Don't worry about it, I run into the same problem all the time, it can be very hard to get nuance across through text.