If you do manage to get the piston to go down, check the sleeve to make sure it didn't get scored. You'll see this as very shiny vertical lines or gouges in the sleeve.
I don't see the need to flush a engine completely before putting it up for the winter. On your very last flight, simply drain the tank, keep trying to start the engine until all of the remaining fuel in the engine has been burned off and pour on the after run oil. Pour it into the carb and into the plug hole and spin the engine until the oil is everywhere. Periodically spin the prop during the the winter to move the oil around and keep everything loose and you should have no problems come spring.

Jesse