"Thirdly, on the point of fastest body speed, you are right that it occurs at the lowest point of the fall."
Agreed... although to clarify with another discussion, it's actually the lowest altitude. Fall is a slightly different concept.
"On a curving or centrifugal trajectory velocity is fastest at its terminus or peak, allowing for gravity, friction, blah blah blah."
This doesn't make any sense. Velocity tends to be slowest at the peak or rise, regardless if there is rotation or not. Many posters here seem to be confusing total energy with the component of total energy represented by horizontal velocity. Generally, total energy is near constant, but changes between potential (rise) and velocity (flight) during the swing cycle.
BUT applied to rise and fall, that puts the fastest body speed right after 4, into 5, or 1 into 2 of a Waltz Natural Turn. NOT 3 into 1 as you pointed out above.
You need to review the formal definition of the boundaries between steps. At the formal end of step one, rise is under way so the body is already slowing. Wheras at the end of step three
"One should not be so technique-bound that one becomes a closed-minded prisoner incapable of considering other methods that clearly work."
But one should not mistake moderate success of oneself or ones teacher with a situation not in need of further improvement with regard to efficiency. Analysis is a reliable tool for guiding improvement, feeling is not.