For my eyes, the most distinguishing identifier is the “shadow” of the left collar on the medal on the right. That, as well as the chin, as you point out, makes me think that this is not a case of incorrect ribbon. The medal is, indeed, an EviiR. If not coronation, then what would be lower in precedence than that.
I think that your theory is the correct one. Knowing that he would soon have his third medal, but not soon enough to have it in time for the photograph, he opted to pin on one of his mate’s medals, probably thinking, “Nobody will ever notice the difference!”.