There are a number of bugs here.
Unclaimed workbenches cannot be reclaimed.
Workbenches cannot be undeployed.
Workbenches cannot be re-assigned, assigned dynamically.
and 2. I assume are known. 3. May be a design decision, but it seems like their should be a fluid number of carpenters and benches and carpenters should complain when their work is blocked because a resource is unavailable. Really, a lot of people should complain about that. 
I tried to reproduce this in r166, but the workbench was automatically claimed by a new carpenter. It might be an issue with old saves? Any specific steps needed to reproduce this?
I am on vacation, if the version is current when I get back, I will play around with this one. 
(r166) Two of my carpenters died. Both of their benches remain unclaimed, can not be undeployed and are unmovable.
Ah, so the steps to reproduce include feeding the current carpenter to the goblins. My previous attempt to reproduce is void…
I have also noticed that deceased citizens sometimes show up in the citizen list. Could be coincidence or could be a single bug creating multiple issues.
No, that’s an old bug. It’s already reported somewhere.