Thinking it over, it seems the underlying problem is that in your takeback really is that you inadvertently turn it partially into a "positive" or "concentric" movement. I think one thing that bothered me was how your racquet face didn't really open up until very late in your takeback, and to me that seems counterintuitive if you were stretching out the arm.
I *think* it'll help just to keep in mind that you don't want the elbow to straighten in the takeback or as you set the trophy. The elbow will more bend more as you get to the trophy position, not lengthen out. And due to that, it will preserve the takeback as a proper stretch as well as prevent you from excessively "raising" the arm and then the hand/wrist.
BTW, Sharpy, did you say you were pushing 80 right now on the serve?
In Sharpy's serve, he isn't getting any pronation at all:
Yeah, the external rotation of his arm is off, so he's kinda slapping the ball down. I'm kinda hoping once the takeback thing is resolved, the pronation will start happening for him.