Do we know if/when Fritz found out?

I don't. I hope he didn't! Perhaps selenak knows, though.

Also, Pleschinski says that Voltaire visited Bayreuth without Fritz, is he wrong about that?

I would have to check, but my impression from Oster was that they went to Bayreuth together, but Fritz went off and did his own thing for much of the time without Voltaire, who stayed behind in Bayreuth. Take that with a grain of salt, as I'm going by memory and don't have time to check at present.