My take on the mole being on the other cheek: mirrored image. No idea why the photographer (or perhaps the publication that released the image) would mirror the image. Sometimes its done to fit a certain look the publication is going for.
As for sometimes not having a mole: Pics could have been taken some time apart at a point where the mole might have been removed. If they were taken at the same time/same photo session (or before/near that time) then I would attribute it to when a photographer will clean up an image (airbrushing) to get rid of imperfections. Since the pics are old, not all photographers at that time would have had the technology to do this for every photo. Michael may have been self conscious about it and had the mole removed depending on the picture?