The DC National Guard would not be able to do anything on I-495 without the cooperation of MD and VA.
I-495 is in MD and VA not in DC.
Wrong - there is about 200 feet of the Wilson Bridge in DC, so they are physically capable of putting a road block there, if they had the legal authority to do so.
Fortunately, they do not, and if federalized, could not because it might come under Posse Comitatus.