I have a L3901 and engage the brush cutter at ~1500 rpm.
To quickly reduce speed of blades, stop tractor, leave PTO engaged, throttle all the way down. When at idle speed, disengage the PTO.
I rarely if ever disengage the PTO while mowing until I am finished.
A pusher fan may help the A/C if the condenser is not plugged, but probably won't affect engine cooling. Make sure the engine fan is centered in the shroud and shroud not broken. There should be rubber or plastic seals between the condenser and radiator to form an airflow tunnel from the...
Temp cycling like you describe sounds like a sticky thermostat, Another possibility is the stat "jiggle pin" may be stuck closed. The stat should dampen out and modulate temperature without excessive swings. Yours seems like step changes. Stat should be replaced.
I have a Honda EU3000i generator as well. Just before we had an auto standby generator installed, we lost power for about a day. With the Honda, I powered 3 freezers, a fridge, and the TV. It used a little less than 3/4 tank of gas