|Since you brought up A Chorus Line, Jane Summerhays always sang Sheils's opening part of "At the Ballet" up half a step. This was first done when she was on the international tour (the cast that opened the show in London after two months in Toronto, and then that company, with regular cast changes, toured North America after the six months in London), and then later on Broadway. I have been told she was the only Sheila to sing it in that key, but I don't know if that's the case.
I used to be under the impression that there were only two different keys used for "Music and the Mirror" on Broadway and the various tours, London, etc.: the McKechnie key and the Reinking key. But I think you may be right that there were more than just two.
There have been cases where a key was changed from the Broadway key for a tour or a foreign production, and then a replacement on Broadway sang in that other key. Sometimes the tour performer came to Broadway as a replacement, as with Summerhays.