Not much point is providing further references if you ignore the one already provided



Here it is again with a better pic. Take two props both with a 30 degree blade angle, one stationary, one wind milling, the wind milling prop has a drag coefficient 50% less!

The drag created by a propeller is a function of the local airspeed over the blade and the relative angle of attack of the blade.

When a propeller windmills the relative angle of attack is reversed, flat pitches typically results in a high angle of attack and high local airspeed, i.e the propeller does a lot of work, and drag on the aircraft is very high.

The engine/gearbox is a red herring because it is not the cause. A free wheeling propeller with no engine/gearbox slowing it down and a very flat pitch will result in a very high drag coefficient, many multiples higher than a stationary one of the same pitch. A feathered propeller will have the far less drag than both these two scenarios.

John with respect I am not arguing with the rest of your post.