Lancman
Newton was a clever chap, but Einstein trumps him.
You are completely, utterly wrong when you say:
"If a hang glider is flying at 20 knots in a 20knot headwind it has zero kinetic energy but plenty of potential energy, I hope"
Kinetic energy is all relative to some frame of reference, hence Einsteins "relativity"
The relevent kinetic energy to an aircraft in flight is that between the aircraft and the air.
That is the only valid frame of referance.
This is easy to see if you consider the aircraft relevant to the earth, or to the sun or to the spiral arm of the galaxy.
The kinetic energy is all about the difference in velocities of the two objects. It is not a quantifiable absolute, rather it is quantifiable between two objects.
In a crash, yes, the energy between the ground and the aircraft becomes relevent suddenly.
You notice that all those that believe this downwind effect to be true are the ones with the highest reliance on the visual external cues. Visual clues which we know are very susceptible to errors in the aviation environment.
The same problem comes up when yacht racing in a river/tidal estuary near the shore. As long as you sail by the water not the shore, everything is fine.
Wings don't get confused by visual effects, and neither do instruments. Trust them.
"To believe that an aircraft is excused from the laws of motion can be dangerous. " is undeniably true, but not as dangerous as a lack of understanding of those basic laws of motion Lancman