Bertholy doesn't answer the biomechanical questions that I pose. I would ask, why the differences and maybe they will never be answered. I don't think you can always respond by saying "Bertholy" said this or "Bertholy" seaid that. Heck, half of the things Moe said that he did were different than what actually occured.
I spent a day with Paul Bertholy and took a lesson from him because Moe told me to go see him. I enjoyed Paul and had a great day. His methods were fun and had what you describe as the "fundamental" elements. I think, however, that there is much more to it than that. There are causes and effects even if you hit all of the fundamental positions.
You say that the differences between swings is "not that much". I am not talking about differences here, I am talking about the causes and effects swinging a golf club whether it be conventional or Single Plane or any variation in between.