I just took a look at the Ben Fox and Anthony Kim sequences on Riggs' site (both are star pupils of his) and have a couple of observations. First, a golfer's degree of shoulder turn is in fact the combination of the shoulder turn around the spine AND the hip turn. For most players, I've been told that 45 degrees of pure shoulder turn is about the max, though Fox and Kim, being young and flexible, may well exceed that. Second, in looking at the Fox and Kim sequences, the most important thing to observe is that both have relatively little hip turn, a function of the right knee remaining flexed and not moving AT ALL away from the target line throughout the backswing. That, I believe, is the primary reason for their short shoulder turn, which I don't think is even 90 degrees: a line through the shoulders points to the ball, not the center of the stance.
As for one shoulder turning meaningfully more than the other, that sounds impossible, despite what the pictures appear to show. As far as I know, there are no joints that permit independent movement of the shoulders relative to the spine. Anyone with medical training feel free to chime in.
Jeff