I've recently been playing around with computer music recording and production, and I can get many of my voice or whatever playing over each other. It's all about using multiple tracks.
My guess in terms of things where Grace played the recorder and sung at the same time was the multi-track thing, and when they played live, someone else would play the recorder for her...or else my longheld belief that Grace is magical is true...it could be. Maybe she plays the recorder through her nose and sings through her mouth, as normal.
Ok, I'm really tired so ignore that last bit there. It's too weird, even for this forum. lol