If the law was changed to separate them from the US Army, they only served inside US borders, and they were allowed to keep their weapons at home, I'd be happy to start counting the National Guard as the militia.
State guards would count as militia if states actually armed them with military weapons.
You can join a state guard without having been in the National Guard or in any other military. It is likely however that ex National Guardsmen are the people most interested in joining their state guard.