If I'm not mistaken reducing RPM to improve glide time/distance only works as long as the prop is windmilling. That means that you have to shut down the engine just before you run out of oil or overheat due to lack of water. If the engine siezes for any reason and the prop stops it won't help anymore, even if you reduced rpm prior to the siezure.
in real life a fully feathered (stopped) prop creates less drag than a windmilling prop, no matter what the rpm.
if its indeed true that the seized engine (prop stopped) in game is the same drag as the fullly reduced windmilling rpm in game, then theres a slight problem in the modeling..
question 1 i do not know answer to: historically, could these planes actually fully feather their props? if so, why cant we?
whatever the case, in game, shutting off the engine stops oil leaking, and stops heating up engine from radiator leak, and therefore you'd want to use the lowest rpm possible (0 rpm being best) to minimize drag for your glide..... (if that was the OP's question
