Let's not jump to conclusions about Mental so quickly; what makes you so certain that he's a mechanic?
I'm thinking, maybe the pianist should block the Lizard? Naturally, as a mechanoid. The Investigator would check him. If there are any doubts, he can come to me. Then we'll have one more or less peaceful night (I think the Knight won't go crazy in this scenario, even if he's not the Lizard). But that's an option if the last execution doesn't go through. Because I'm voting to execute the Lizard.