Yes, Master Paintstick propellers are nearly indestructable, good for beginners. An 11x5 or 11x6 will give good thrust, while leaving enough ground clearance for learning to land. A 12" propeller will come too close to the ground, and will be too easy to hit in a slightly rough landing. Beginners' first landing attempts are a little more than slightly rough, if you get my drift.

As long as the nut threads completely on to the crankshaft, you have plenty of crankshaft. Don't worry about only having 1/8" of crankshaft sticking out the front. It serves no purpose once the nut is tight anyway...

You probably should break the engine in with the spinner installed, especially if you have an electric starter. The cone on the electric starter won't grip the nut properly.

Don't worry about using the plastic spinner on this engine, but when you start getting into larger engines, particularly 4-strokes, use aluminum spinners. A backfire on these larger engines will sometimes cause a plastic spinner to shatter and spread shrapnel in every direction. While probably not fatal, a piece could get in your eye and do serious damage. In any event, it's not a pleasant experience, so just don't do it, okay?