I agree with most of what you said, I just have a slight different take on it. Is okinawan Sanchin a combination of a few styles? It's a good possibility however Kanbun stated many times that he did not change anything from the way he was taught. He was adamant about passing on the art exactly the way he was taught. Now if I combine that fact with the fact that two other arts from a different source look dam near identical, I would have to say the mash up would be from the Chinese not the Okinawans. When I eluded to "elaborated ending" I am referring to the differences between the Okinawan sanchin and the multiple versions of Chinese sanchin forms. All three okinawan versions do single arm thrusts, 3 double arm thrusts and Tora guchi. While most Chinese forms don't stop there. Usually there are more blocks, strikes and other moves that I have no idea what they are..lol... then end with the Tora guchi. These extra moves tend to be done with less dynamic tension and at a different tempo. And I find it interesting that every different school has different moves but maintain the foundation of the double thrust strikes. As far as Miyagi doing backward stepping, yeah the official story is the he invented the backward stepping in order to have students develop those muscles ...yatta yatta yatta. But since it is almost universal within the Chinese versions I think it's more like he saw it while in China and decided to adopt the practice. He could have also learned it from Kanryo but then one would have to ask, why he didn't teach it sooner. This is just my take on it and I'm open to new interpretations if the facts lead in that direction.