First off, welcome again. Good to see you posting.
Your question(s) seemed to be open ended. By that I mean it seems that you're going in a specific direction for a reason(s), without saying so. Perhaps it's because of an observation or a technique used or suggested by someone - I'm not sure. Can you be a more specific? On the other hand, I can be completely off base on this.
- is it a good thing to raise the pivot leg?
- should the pivot leg be straight out, or belt high up, over the head, etc?
- does the back leg do the same thing for every pitch - even the knuckleball?
- what about those pitchers that stay upright during and after delivery?
Do you see where I'm going with this?
Also I might add, the body in motion on a slanted surface, or a surface that's in so-so condition for pitchers, has a multitude of scenarios that's really hard to nail down by focusing on just one body function.
In this regard, I'm not all that familiar with youngsters and what they have to deal with - so, my observations on that is somewhat useless.(I think)
So, your question about shoulders is kind of hanging out there by conditions that (might) come before that, perhaps not after. Again, this can be a question answered by others better than myself.
By the way, your question is outstanding. I tried to correct a guy who had this mule kick in his back leg - a lefty at that. (lefties are not my favorite to coach) He believed that his mule kick actually gave him that extra "umph" to throw harder. Finally after months of working with the man, and convincing him that he'll hurt himself (for his age) if he continues, he glides through two weeks of spotless preseason form. First shot on the mound in a game, doesn't he go right back to his old self - bingo, groin sprain.:glare: So as much as the rest of a pitcher's progression and posture is not all that bad, certain things enter the mix that are self-convincing for reasons other than posture and form alone.