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- Ghost expires/gets slabbed/buried, and "dead" and "killed" are set again?
This is correct.
What about husks? Do they keep their souls?
I haven't looked into this, but I think that the changes observed in husks were just regular old syndrome effects, so the soul data is probably left intact.
It was last updated in 2011, though, so it's possible that it was fixed unintentionally at some point since then. If anyone can reproduce it in 0.44 or so, that would be helpful.
I tested this by inducing a fell mood using the "strangemood" plugin and teleporting a ghost into the same room as the target unit. The ghost was subsequently murdered, so it appears that this particular bug hasn't been fixed yet. Note that no corpse is generated since ghosts are produced with all body parts missing, so the mood material requirement remains unfulfilled.