RE: Stuck Prop nut
Take off the backplate and wedge a Bic cigarteet lighter in there to stop the crank from rotating (heli guys do this to put the clutch on). If you use a bolt or anything that contacts the piston to "lock" things up, you are risking screwing up the piston.
I'd rather take off the backplate than have to replace the piston and possibly a bent con-rod, or messed up wrist pin, or replace the crank pin or...