Being Duke means appointing Lords, though, and that loyalty/oath is pretty big, to a normal player.
I've found it safe to assume that every Duke has a powerbase of loyal vassals to draw on. Convincing them to join a seperate cause is the only way to really undermine that duke, but it's extremely difficult to Do so.
Appointing power only matters if you have regions in your duchy. Sevastian's vassals being Lurian (I suppose) probably helps their loyalty to him, but I've otherwise not found it to be that hard in most cases to convince lords to change duchies.
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. 
All the anti-Sevastian people have to do is ask Morek to conquer regions from them which are under Sevastian's power, and then have Morek give them back when the situation is deemed satisfactory.
