Based upon his earlier knowledge check, Lawrance would be quite confident dumping the vampire over the side would be the end of it.
You have her resting place, she cannot regenerate without it, even if the stake were removed, and the castle is traveling much, much faster than she can in mist form.
So either her body is destroyed on impact and the mist makes a futile effort to reach the coffin, being destroyed when the sun rises, or her body is not destroyed, and remains paralyzed there until the sun rises.
The most likely scenario in which she survives such a scenario is if a sympathetic party comes along and heals her with a necrotic spell. Which seems a bit of a stretch.
Now, all of that stated, letting her rest in the control room as Velon suggested should be safe. Once again, she's not going anywhere.
EDIT: I should point out that you haven't determined what the bathroom pit does with things dropped in there. You just haven't seen them again.
Edited: daishain on 19th Dec, 2017 - 9:30pm
Now that we have confirmation she could NOT catch up the castle, I am a little more amenable to dumping her. Before doing so, Poljen would insist on making sure the stake is securely in her, sticking all the way through the chest and back.
But I still prefer the certainty. Again, Poljen will trust Velon's judgment.
I am fine with that. We need to find the fastest route to the sun once we know it is up. Or if someone interrupts our rest, yet again. So let's haul her carcass down to the control room, and Poljen will sit his rear end down once more…