The AC runs when you are in Defrost mode, even if the AC button isn't lit, even if you have the temp set warm. That's how pretty much every car build since the late 90's has been. AC dehumidifies, that's why car manufacturers link the AC and Defrost.
I don't use the "Front" Defog/Defrost. I can do the same with just outside air, heat (increase temp using the selector knob), and air outlet selection (windshield). Maybe the hatchback is different in this respect... other than the "Front" Defog/Defrost button, I can enable or disable the A/C separately by pressing the switch.
A little over 5 weeks of owning my Prius, I played with most of the techniques to get the most mpg for the first few weeks. But I found that for my drive, 135 miles r/t 5 or 6 times a week for work commute on So Ca freeways it was not a lot of fun so Ive taken to driving the car like a normal car and probably even accelerating harder in it than my other cars since it has much less power than what Ive been driving for the past 7 years. Im still getting an average of 45 mpg, 85 mph, accelerating with the pedal to the floor in Power Mode and the A/ C on at 74 degrees. Thats 2 twice what I used to get and Im using Regular gas. So Im happy , this car will save me over $ 3,500 per year
Hypermiling with 17s is more difficult than it is with 15s and the results are less impressive. When I would run 17s I would give up on hypermiling and I would just drive. Obviously my mpg would drop but the car was simply more fun to drive. I can totally understand why you gave up trying.
Interesting, I can't get nearly enough airflow onto the windshield without using the Front Defrost button. Then again, maybe I'm a heavy breather.