My father was a P-51 fighter pilot in WWII. Another P-51 flown by a buddy of his suffered a "runaway prop", and as a result of that, the plane could not maintain its normal high speed, and in the end, the engine overheated and the plane exploded over the English Channel. Luckily, the pilot, Major Foy of the 357th FG, had noted excessively high engine temperatures, and bailed out just before it exploded. What would cause a "runaway prop" of the variable prop to create a loss of thrust and a high engine temperature?
A "run away prop" is caused by a failure of the pitch control mechanism of a constant speed propeller. Feathering turns the prop blade as close to 90 degrees to the wind as possible, minimizing the effects of the airstream. This obviously reduces drag and is used when an engine is shut down.
A run away condition is created when the propeller malfunction causes the blade to move to the "fine" setting, which is used at slower air speeds. It is the opposite of being feathered in that now the prop blades are much more angled to the wind as in a windmill. The force of the airstream literally now turns the engine to excessive rpm. The prop is at the wrong pitch for its airspeed, much like a truck dropping into low gear at highway speed. The engine rpm redlines and causes overheating, while the drag of the improperly set prop slows the plane down.
Thankfully, the pilot was aware of the danger and was able to get away.
The prop regulates engine rpm using a flyweight governor (functionally similar to the ones on steam engines) which drive propeller pitch coarse or fine as required to keep rpm constant, regardless of throttle setting. The governor either controls an electric signal (curtiss electric) or engine oil pressure (hydramatic propellers) to provide the driving force to rotate the blades.
The aerodynamic loads on the blades are always trying to drive them to full fine pitch and the servo forces varied by the governor push against that. If the governor fails or the oil supply to the prop hub fails such that the governor is not able to vary the blades against air loads, the blades will naturally relax to the full fine position, which unloads the engine and allows it to overspeed; how badly depends on the throttle setting.
So if a prop runs away, you'll be cruising along minding your own business and suddenly the RPM will surge past the redline until you yank the throttle back. It's not that different from pushing in the clutch on a car while you've got the pedal down. You'd better let off on the gas post haste if you don't want to destroy the engine.
Overheating is something else. A runaway happens very quickly, whereas an overheating engine is more of a long term problem. Not sure if the overheating was directly related to the prop problems. If you'd had a runaway but were able to reduce the throttle enough to keep the engine at or below redline rpm, I'm not sure why you'd get overheating with the power cut way back. It's possible that to avoid descending, he kept enough throttle on to maintain altitude but in doing so the rpm remained well above redline and the extended overspeed operation overheated the engine (Mostly from the excess heat generated by the too fast piston movement).