"I can assure you that Anne Lewis said that Propulsion comes from the knee and the thigh."
Well, she's wrong. She's a dancer, not a physiologist.
"I doubt that too much lowering would be the problem."
Lowering unmatched by proportional horizontal movement usually implies breaking the body position and invading the partner's space. Also, lowering unmatched by horizontal excelleration means energy is wasted rather than conserved by conversion from potential to kinetic.
"To get back to body flight. If I leap from one stepping stone to another. I think I would bend my supporting knee and push."
If you leap, sure. But that's not a ballroom movement. Ballroom movement does not have rise and fall cycling per step, it has it cycling over a series of steps. The closest thing in ballroom would be various quickstep tricks, but even then is quite different from stepping stones.
"I believe my foot would lead off."
I sure hope not! Outside of tango, everything is body-first.
"The body flight must come from the propulsion of the standing foot which propels all of me."
You are confusing body flight with body speed. Body speed can come from the legs, yes. But body flight is the body speed remaining from the previous step - it is what you already had before you used your legs. If your motion is not continous across the steps, such that your body would keep moving even without leg action, then you do not have body flight.
"If I tried to move my body first I would never make it."
Ironic, as that's the only acceptable way to dance the swing dances!