What Jack doesn't seem to get is that declaring war while within Abhainn's borders would be the stupidest thing he could do. He's rash but he shouldn't be stupid. Doing something so rash and openly hostile while in the heart of the enemy kingdom is basically asking to have Jack be taken to the gaol at best. Would he declare war and make a run for it? That would be like going into a police station and shooting a gun in the air. What king would turn a blind eye to a madman murdering his people and saying "WAR'S ON!"? If he really wanted to declare war, it would've been a stronger message of intent to refuse both the wedding and showing up in person. There's no really good scenario for Jack if he murders innocent foreigners in their homeland and/or declares war while a guest of a sovereign nation. Kings aren't above the law when they're not within their own borders. 

Increasingly it seems Rhoslyn is going to have to be the one that does the duties.