Heat and temperature are related but not the same.
It takes a pretty long decent to over heat brakes.
By pumping the brakes more heat is released to the air. Even on disc brakes a small gap is way bigger than no gap. It does not take much air flow to carry off the heat. On a hot day even the slightest breeze makes a difference. Radiant heat will find its way out with no air flow.
Under harder braking you will generate less heat as it wears the brake down more and that absorbs some energy.
As the brake gets hot it has less friction so more of the kinetic energy goes to heat then temperature.
Alternating the brakes allows you to pump the brakes and maintain a constant speed. You could pump both at the same time for equal cooling.
A slower decent will have less temperature because the brakes have more time to cool. And the heat is generated at a lower rate. There might be window were medium braking is more than light but if you slow it down then you can pretty much always control temperature. I loaded truck will creep down a long hill.
If you feel a brake starting to fade (less friction) stop and let it cool off.
On rim brakes you can heat up tires.
On disc brakes you can boil the brake fluid. Naturally a bigger disc will have more cooling.
I will use the rear on a longer decent as the primary to have some reserve in the front.
Don't go into an aero position so more air drag.