RE: physics of the downwind turn
I've had similar thoughts. Don't have much time these days to devote to thinking it through, however.
Getting the same clear answer in both frames of reference is essential for me to be happy with my own understanding. Something is out of whack in the all the explanations until the answers are correct in both frames of reference. Can't say I'm there yet....
Impatience with the question isn't an answer.