hybrids have heavy duty starter motors for this purpose. I'm not sure how robust the fits is. Maybe when the hybrid fit comes out we can retrofit those starter motors.

 

I would leave it in D unless you are parking or backing up.

Shutting off and on has no effect on spark plugs. And "ignition" is a process, not a part that can be worn out.

Doing that will put an extra wear on starter, batter, and the flywheel. Don't do it.