If he marries a Corsican, then that only mollifies one set of Corsican power-brokers whilst the others get offended because he should've married one of theirs instead. Much easier to marry outside of Corsica and get a foreign notable like any good King should.
What's the odds of losing any more citadels? Slim to none? Though right now if Theodore can get the rebels together he's got the kit to make something happen.