Mafia 83. Doctor Who: Aliens in London

I don't mind executing Ment, but Yashur seems more suspicious to me.
Ment remains silent and doesn't reveal anything.

And yes, if we do execute him, won't he continue to regenerate?

Most likely, he will, but the regeneration will be delayed by one phase (as if he were killed). There's a plus to that: even if Ment is a Doctor, we won't permanently remove a strong town role from the game. However, we'll never know his role for sure.