[Par71]

Law of the Flail applies to just PA2 (a Flail doesn't gave a roll/tumble), correct?

That is a misconception. A common flail (with a thong) definitely shows turn and roll. The golfer's flail is "only somewhat flail-like" (because of the way the hinge pin works). But even the golfer's flail can Cock and Uncock and Turn and Roll (2-K). Study pictures 2-K #4 and #5: They both represent the golfer's flail.

And PA2 precedes and causes PA3 (Roll TRANSFER Power), correct?

PA #2 may precede PA #3 or not (Sequenced Release vs. Simultaneous Release). But PA #3 may not precede PA #2.

PA #2 does not cause PA #3. Nor does it release PA #3.

So even if PA2 is spent, PA3 is still doing it's thing. So, even if you reach full extension/uncock at impact or bit before it, the sweet spot is still up to speed as PA3 (and PA4, and PA1 if you use it, as a matter of fact) isn't spent until after impact?

The idea is that more force is applied to the ball when it is struck with an object that is still accelerating at Impact, rather than with an object that - although moving with speed - has already stopped to accelerate. That applies to all Power Accumulators. So, for maximum power, no Power Accumulator should be fully expended before Impact.