I think it’s a montage. The draped pearls hang in too round a shape and the pearls around her neck are too perfectly placed. Also the Medici collar is a Queen Alexandra thing not Queen Mary. The crown is correct for QM (QA’s crown is rounded at the time whereas QM’s is more pointed). I would guess it was done after she became Queen when there were few gala images of her so they put a head on QA and added some pearls. The royal order of Victoria and Albert seem legit.

Queen Mary Coronation Dress

Edit

Looking at it again I think it might be a real photograph portrait of Queen Mary that has had; crown, Medici collar, necklaces, brooches and swags, robe and over sleeves added to it.

Take a look at this humdinger