One basic point to make clear is that the power is force times speed. So at a lower speed one has a higher force at the same power level. Confusing power and force is a mistake sometimes made by beginners.
The basic idea behind the vehicle is to take up power from the wheels and use this power to drive the prop. The prop than produces force relative to the wind.
So it looks a bit like the fake over unit devices, but there is a twist to it, as the two parts have different systems to act on:
With the vehicle moving in the direction of the wind, the speed of the wheels to the ground is relatively high.
With the vehicle moving with about the speed of the wind, the relative speed between the prop and wind is relatively low and with a speed lower than the wind speed the wind is even driving the prop. So even with a not so good efficiency of the prop it is possible to create more forward force ( = power / speed) than the backward force from the wheels. So the net force can be forward.
The energy comes from slowing down the wind. This is seen as the backwards force from the wheels. With the fan blowing in reverse this even works when moving faster than the wind or also when moving at the speed of the wind. So there is no dead point or special case when the speed is the same.
The difficulty in actually making this work comes from getting good enough an efficiency from the prop and gears. No need for over unity, just not too small (e.g. < 30%) to not overcome the friction forces and parasitic drag.
One can test the easy purely mechanical (no aero dynamic) analog. The nice point here is that it does not depend on speed so much and can be done hands on on the desk: Have a spool of thread with the outer rims larger than the diameter where the thread is. To improve the gearing ratio maybe add some rubber rings to the outer rims. With the spool on the desk slowly pull the thread horzontally. The thread is the analog to the wind.
With the thread on the top side the spool moves slower the thread is pulled, so nothing funny happening.
With the thread on the bootom side the spool moves faster than the thread is pulled. On a slippery surface it may slip.
This is where you (all) make the mistake. No wind energy can be extracted above wind speed when vehicle drives in the same direction as wind direction or opposite to wind direction.
Traveling in any other direction of course is not a problem to get energy from the wind at any speed as you always have access to wind energy.
Using force and speed (while not fully understanding how they work is what creates the wrong conclusion).
The main reason I used a wheel to replace the propeller is so that the misery of how a propeller works is eliminated. Propeller is nothing more than a wheel for air.
But why not use an example.
Say there is no wind and vehicle travels at 5m/s and say kinetic energy of the vehicle at that point is 500Ws and a break of 10W is applied for 1 second.
Vehicle will end up at 490Ws and say all this energy is then transferred to propeller that is 70% efficient (fairly good propeller efficiency).
Now there will be just 7Ws available for the propeller and no matter how you set the angle of the propeller blades to get more force and lower speed or higher speed and lower force you will still end up putting back 7Ws in to vehicle thus you end up with less than you started 497Ws vs 500Ws so lower speed.
Hope you fully agree with my example above and understand that 7Ws is all you have available and no matter how you use that you will not end up with more.
Now if there is a wind speed say 20m/s and vehicle is above that speed say 25m/s and you break the vehicle with 10W for 1 second then you have available 10Ws with a 70% efficiency means 7Ws to use for the propeller. Since wind speed is in exact same direction and lower than vehicle speed the wind can not contribute with anything to the equation thus you again end up with a deficit no matter what force vs propeller speed you select.
And I used energy instead of power as maybe it is more helpful but for power it will work in the exact same way.
Using forces and speed will just complicate things for you and you will come to wrong conclusions.
In my wheels only example there is a G (generator) wheel and a M (motor) wheel and say there is a 2:1 gear ratio that means the generator wheel will make 2 full rotations while the motor wheel will do just one.
So say treadmill speed is 5m/s and say you apply 10W of breaking power that will mean a breaking force of 2N now if you look at the motor wheel you will see that spins at half the speed 2.5m/s but twice the force 4N assuming ideal vehicle where all 10W from generator are available at the motor wheel.
Now if you look just at the forces you will think that since there are 4N at the motor wheel and only 2N breaking force at generator the vehicle will advance forward but that will be the wrong conclusion to make as the force at the wheel is not transferred to the body but to the other wheel.
Say wheels are mechanically connected with a chain then those forces will be transferred to the chain not to the vehicle body and so since power is equal on G and M wheels the vehicle will actually not move in any direction. The same happens with the propeller based vehicle as propeller is just an air wheel.
I will expect here there are more electrical engineers and they understand that power is a more important unit than current and voltage (speed and force).
And as far as vehicle is concerned as soon as it is at same speed as the wind or above wind speed (same direction) is not accessible at all to the vehicle.
You can imagine air as small fling balloons that if they have higher speed than vehicle can push the vehicle by bumping in to it and as soon as vehicle has higher speed all the balloons that vehicle will see will come from the opposite direction of travel thus not only they will not help but they will be counterproductive.
This is a long reply and maybe not very organised but I hope you take the time to read. Especially the part with the example.
You can not claim that if you use forces and speed in your calculation the result will be different than if you use power as if applied correctly the result will need to be exactly the same.