Aircon blower fan motor's speed ( voltage ) reduction via resistors, wastes electricity & produces dreaded heat.

Not to my knowledge. Sometimes fan controls (thought perhaps not the heater fan - too large) are PWM but I've never seen a buck used. There really isn't a reason to spend the money.

In the grand scheme of cooling a car a couple hundred extra BTU of sensible heat is not going to make any difference. The A/C unit itself is 24 KBTU or more. (Enough for a small house)

You are doing calculations on the basis of V=(Re+Ri)I where Ri is 0.8 ohms and Re is 0.64 ohms- your calculations are on the basis of a stalled motor-not one that is turning Now consider that in fact you have V=(Re +0.8)I +E where E is the back emf produced when the motor is turning. (and much of the 7.5V measured is attributed to E), you will find that the resistance added will be quite a bit larger than 0.64 ohms (say 6 ohms)and the loss will be quite a bit lower-say in the 10 watt range(without knowing the current, this is ball-parking estimates at best). Yes, speed control with a resistor isn't the most efficient way to do it but for motors of that size- but it is cheap-it isn't worth spending more- the cost/benefit ratio for both the car maker and you isn't worth it. I suggest that, in addition to the voltage measurements-measure the current. Then you can get a handle on the losses and the total input power. While you are at it-check the panel lights and/or radio as heat sources.