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Yeah nice explanation. I liked the qualification of detached flow, I always imagined that the stream of air detached from the car like a ball rolling across it would. The fact that it is stationary or backwards moving air (relative to the car) is useful. That helps me to understand that by taking some of the energy of the faster moving higher up streams the difference in speed is not as great so less air has to move and less force is applied on the vehicle. I'm not sure if I understand the momentum analogy. Did they assume that the high pressure on the front side would equalise with the low pressure on the back side but the surface friction prevents the two pressures from equalising? I'm imagining something like two connected balloons with equal pressures in each (perhaps one at the front and one at the back of the car). The car is effectively pushing air into one of the balloons (the front) leaving less pressure in the other. These pressures can't equalise because the friction between the two balloons slows the transfer of air so it can't 'catch up' to the rate that the car is pushing it. Is that right? You can ignore the balloons that just was to help me consider the two separate areas of pressure...
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