Yes you are correct about the reasons for feathering. Primarily it is a means to reduce drag, in terms of restarting mid air it depends on the aircraft type. For my type when a relight is attempted the propeller is moved out of the feather position by the unfeather pump. The windmill effect you speak of actually is used to provide initial acceleration (usually done on the ground with the starter generator). This is only ever done at altitude where the effects of the windmill are safe. In terms of auto feather this is engaged on takeoff and landing but disengaged in the cruise.