John,
I agree with the basic thrust of what you are saying, in that models of the universe don't have to be perfect, they just have to be 'fit for purpose'.
Certainly, I often preface part of a lesson by saying something like:
"this is a simplification, we haven't taken account of x,y,z... etc, but its good enough for our purpose and gives us a working approximation of what actually happens"
Additionally, there certainly are a bunch of dangerous things about turning downwind at low level, but these are related to gusts, shear, and incorrect perception of balance.
But this muddling of the frame of reference that is the crux of this thread is not needed to explain these hazards. Rather it provides a model that suggests behaviour which does NOT actually happen in smooth air, and suggests that turning downwind ALWAYS needs less power. This is not only wrong but it is dangerous.
As such it fails your own criteria.
Incidentally, you've said you don't want to get into the theories, fair enough. But I have to say that we are not talking about a complex application here. It doesn't need a degree in aero eng or Einstein. It is basic Physics; namely definitions for displacement and velocity. Thats all. Educationally speaking, long held basic misunderstandings are the hardest to shift.
pb