Ok then, let's attack the boat with Mortus fireballs. Let's save the bombs in case we need them as a last resort, unless they are throwable and can be readied and thrown in ~2 minutes? If we wait too long, she'll detect Sybo and Mortus and probably attack herself.
Anyway, I'd say we fireball the boat, and start fighting whomever gets off to flee. We fight Etelka, and if things go sour, retreat back into the warehouse and use the bombs/crate traps to catch her if she pursues.
If Sybo connects Mortus and the fireballs, we can deal with that later. Admund won't have sympathy, as he hates necromancy. (Remember he had a dream about the necromancer, and felt like Morr was happy about it.) Sybo was going to up and flee town anyway, we fixed his problems for him. I doubt he would "go rogue" in the middle of the fight.