All very interesting. But experience has shown that if the propellor has stopped turning, for whatever reason, unless you have plenty of altitude, the field will select you. Good luck.
From the various comments it would seem that there is not a lot of real world difference between the windmilling condition and the prop stopped condition
Rotating prop= requires energy and stopped prop=parasite drag.
If it ever happens and I have the altitude where it make sense I will stop the prop.
Just for the sake of the discussion an assumption:
For my imaginary airplane the glide ratio is 10:1.
The stopped propeller gives me a 10% increase in glide range.
Glide range at 2000' altitude is 3 miles ( 10 x 0.3 miles)
Glide range at 2000' altitude is 3.3 miles with the propeller stopped.
Barely the distance of the field I'm aiming for.
Glide range at 12000' altitude is 20 miles ( 10 x 2)
Glide range at 12000' altitude is 22 miles with the propeller stopped.
Two miles is a significant difference so it will be worth the trouble of slowing down to stop the prop, even if you lose some of your gain due to slow flight or stall to stop the prop.