Solari: with the current situation, your plan would only work so far. A new Duchy in BR is impossible, from what I have seen, although I could very well be mistaken on that. Sevastian holds BR and his Ducal seat with a death grip.
Malus pretty much handed it to Sevastian on a silver platter, it'll take more than utilization of game mechanics to convince him to give it up, if anything can.
Exiling a Royal Duke = a very dangerous act. In coming days, we shall see why.
So that everyone is on the same page, I'm not taking sides in this (OOC). IC is another matter. I imagine that from the perspective of Malus (and MANY), Bowie is the Troll/Scourge of Dwilight, and anyone that hitched his or her horse to that wagon is a short-sighted fool. Entertaining? Absolutely. Prone to firing too early and too often? Unquestionably.
Having said that, I agree that Swordfell's unique geography poses a problem for all involved. I was offering generalized advice. I honestly don't know how I boule resolve the situation in Swordfell, short of annexing it if I were Morek and creating an Astroist colony out of the faithful across Dwilight.
Hint: you may find support for this idea in the unlikeliest of places.