Uh, exiling him was an act against his duchy. Your logic is flawed Pike.
Sevastian doesn't try to pretend he's a ruler. He leaves the majority of decisions to other people. Within his domain? That's a whole different story. A Duke's word is law. As long as that Duke remains within realm laws, he is able to do as he wishes. Look at history.
As for Sevastian's Ducal Sovereignty, I hate to tell you, but you are being forced to accept it. I maneuvered very carefully to get my character his royal status, and to maintain his ducal seat. Exiling him and treating him poorly won't make him leave. I figured that was clear to you by now. No matter how it ends, BR remains with House Guile, and that is a win in my book.
Options arent limited, you just aren't looking for the right ones. I'm not stupid enough to tell you how to go about it either.
No worries about the swearing thing. I just want to be clear, that I as a human being swear worse than any sailor that has ever lived. I refrain from doing so on the forums to the best of my ability, and completely abstain from vulgarity in game. Insulting things? Sure. Vulgar? Nope.