i'm not sure about the backing track. i hadn't heard it was lost or anything and i hadn't heard about it first being done in Detroit. I believe the opposite was true with The Happening. the first version was done in LA and I think that was the one used in the film. but the song released by motown was re-recorded in Detroit.

as for the backing vocals, it has been said that M and F recorded their vocals first and then the next day the A's were added to the mix, pretty much covering up the girls. George and others have said, i'm pretty sure, that M and F are still technically on the recording. but pretty much can't be heard whatsoever.