The only question I know the answer to is - the tickets are free, so they are not depending on ticket sale revenue to keep the theatre open. That said, they could probably only seat half the place and there would still be a decent reaction to the show, so I would skip the plexiglass thing. The theatre is too beautiful to mess up. Also, I'm pretty sure there are more than 8 people needed to put on a show. Hell, there are 8 people in the band (maybe...) - and if we go back into the theatre, we need the band!
I don't think NYC is even thinking of opening anything up again soon, so this won't be a pressing issue (unfortunately).