So you agree with the mass flow equation? (Except that you invented a negative mass flow )
But then you changed the kinetic energy part? For some reason.
Your formula is correct in the car reference frame. And in the car reference frame the car has no kinetic energy. Zero, nada, nothing. However the ground is FULL of kinetic energy.
It's much more intuitive to look in the ground reference frame, where wind power is what I said and the car has kinetic energy. And the car can get energy from the wind.
You do have a big problem understanding energy conservation so not quite sure how I can explain anything to you as all of physics depends on understanding this first law of thermodynamics.
Saying things like more than 100% efficiency is possible and "However the ground is FULL of kinetic energy."
Changing reference frames if done correctly will not change anything so same result no matter what reference frame you use.
So you can not say that my equation is true in some reference frame but not in another. It is either true in all reference frames or in none.
As for negative sign for power or flow it just shows the direction.
When you charge a capacitor you have one direction for current and opposite direction when discharging. You need to differentiate between the two with a sign.
Here you are either charging or discharging the stored kinetic energy of the vehicle.
A vehicle stationary relative to ground will have zero potential energy relative to ground but if there is wind it will have some potential energy relative to air.
And at the other extreme a vehicle traveling at wind speed will have zero potential energy relative to air but some potential energy relative to ground based on speed relative to ground and the mass of the vehicle.
Without using energy storage the vehicle (no matter the design) can only have a speed relative to ground between zero and wind speed and nothing outside this range so no negative speed (direct upwind) and no higher than wind speed.