Is it wise casting a drag queen as a villain given all the anti-drag queen sentiment?

Gosh, what a loaded question! I think it ultimately boils down to a question of representation - these things shouldn't be a problem in isolation, but they can be a problem if it's part of a greater pattern. It does sound like Davies is intent on weaving queerness throughout all aspects of this series, so that will probably make a difference.

I was convinced the lost chord was going to be the mystery chord that opens A Hard Day’s Night.

I had the legendary chord from "A Day in the Life" in mind, which of course isn't really possible on a single piano (at least, not to its full effect).