If you read your original question the temperature is a major issue, and there is a constant energy spent just to stay cool. Also going faster will in fact make it easier to stay cool, a couple mph of wind on a hill is still better than standing still. So my guess is that in your 100F heat a reasonably fast pace could be better than a slow pace just due to energy for cooling issues.
If the temperature was something non-hot things would be pretty close and probably some experiments would need to be done to see which factor dominates. There is a constant power needed to "run" the body just sitting on the bike, the efficiency of the human energy system at different levels of exertion is surely non-linear, it takes more effort to stay balanced on the bike when going slower, etc etc etc.