Ok, so if the user is stationary in a 20 km/h head wind, how much power is required to maintain this position? If you come up with any answer other than zero, you can't explain how a brake works. The brake can maintain this position into the 20 km/h wind while dissipating no power. A rider can do the same thing by simply standing on the pedal preventing it from rising up. One foot on the ground for balance, one foot on the pedal to maintain position. No power transfer. ZERO
You seem to have already agreed that this is correct. In that case you must agree there is no power transfer at the wheels for a stationary bike/car/blackbird and that the equation that predicts power at the wheels based solely on the wind relative speed must be wrong.
If there are no sort of brakes it will require around 42W. A brake will
anchor the vehicle to the ground thus no work is done on the vehicle.
A rider with sufficient weight standing on the pedal will still be a form of brake. It is a gravitational based one but still a brake.
There is potential wind power that can not be used because of the brake but there are 42W available at 20km/h wind speed and that 0.408m^2 equivalent area.
The equilibrium state will be the bike being pushed at 20km/h relative to ground so that there is no more force on the bike and no potential energy relative to air but there is now a potential energy relative to ground based on vehicle weight and speed relative to ground.
Without any energy storage the bike can be between zero speed relative to ground if anchored to ground and wind speed if there is no friction loss so not anchored to ground.
The vehicle can be anywhere between this two speed directly down wind at wind speed relative to ground and zero speed relative to ground and at this ends it will have potential energy storage relative to ground or to air.
This kinetic energy while it is a form of energy storage can not help the vehicle get outside this speed limits if it travels directly down wind at all times.
This kinetic energy can be used if vehicle travels at an angle to the wind direction that is how a sail vehicle can exceed wind speed.
In case of blackbird direct downwind version pressure differential is used to exceed wind speed for a limited amount of time.
In case of blackbird direct upwind version elastic and or gravitational energy storage in combination with stick slip hysteresis is what is used to drive at any speed (limited by frictional losses) for any amount of time.