But, they all seem to have destroyed the keys every time a servant died as a group. So a fake dead servant probably no longer has a key?
"The group sealed the room at the same time they left. No culprit joined the group as they left. Also, all of the seals on Natsuhi's room, the dining hall, and the mansion itself will not be broken."
This means even with a fake death in T1/T2 they can't be a later culprit. So if you're assuming Jess is killed be someone who's supposed to be dead we can eliminate everyone from those groups.