OB Left
I actually encourage detailed posts from forum members. I am not discouraging HB and 12PB from posting a detailed post on their viewpoint(s).
However, I am very critical in my analysis of posts, and I parse every sentence in a post for its logical coherence in terms of cause-and-effect relationships. If somebody believes that a 2nd hip firing and 2nd shoulder firing in the late downswing (just prior to impact) can increase ball flight distance, then I expect them to make sure that their argument is intellectually coherent and concordant with established TGM beliefs.
GBD
You wrote-: "I am not convinced that the arm rotational velocity increase much between green diamond and blue circle (time point for max arm speed)... the arm speed trace seems to plateau at about the same time as the shoulder rotation...approximately."
I am surprised that you harbor that belief. I have always believed that one needs to release PA#4 efficiently so that the arms swing freely across the body into impact. John Jacobs in his book "Practical Golf" strongly emphazises a free flowing arm swing. The arms should not outrace the torso, but they definitely shouldn't only travel as fast as the torso rotates. The arms should surely travel faster than the upper torso in the late downswing.
Here is a photo series of Ben Hogan's swing. Note how the left arm
progressively separates from the chest wall (and hands move further away from the right shoulder) during the downswing.
That's what I call an efficient release of PA#4.
Jeff.