Nice post Linear,
When I have a kid going good I leave him alone more than ever. Had a senior last year that came out for ball for the first time since he was 14 and we put him in the 45 slot, preloaded and he contributed many hits.
Our local HS team has a mixture of styles. About 3-5 arm action guys, a great Olerud type LH kid that hit several HRs as a 9th grader that just slides the bat out of that 45 slot and the barrel is all you notice moving, and the rest are solidly in the 45 slot at set up but I would not call great hitters.
I don't change any swings that are productive. I do find some that are struggling and IMO you just can't fix all of them with the lower half only.
Lastly,
I must question the implication that any of todays great hitters perfected the middle and then added upper body arm and hand action as icing on the cake applied at some later date to go to the next level for bat speed.
These is no evidence in the Science of Hitting that Williams supported or suggested a staggered system of learning to hit.
There was no evidence of this in Gwynn's book and I have seen no early Bond footage that their swing was a transitional process training the hips and progressing to include arm and hand action later.
There is no evidence in Dusty Bakers book either
IMO...The reason is that it did not happen that way for any of those guys nor can anybody give any supporting evidence that it did. The first written material supporting the superiority of that teaching style came from you know where. How great a contribution that really will be is still a question for me personally.