I see what you are saying. But I'm not sure your depiction of the operation of usurpation is correct. The sovereignty of the several states still does exist. The 14th amendment permits office holders elected by insurgent US citizens to essentially wear two hats: One as a de facto US subsidiary office holder, and one as the de jure public servant for the state's national government and it holds them harmless for their participation in the rebellion against their de jure republic.
And while it is true that the US is a state by certain definitions of international law, it is not a country or nation by the same standards. DC has never been granted the status of "statehood" and is therefore not part of the union under the constitution. It is simply impossible to remove something from a whole which was never part of that whole to begin with. DC secession is a pointless exercise.
The remedy for all US citizens is to correct their nationalities and become nationals of their respective natural states, which still exist in lawful constitutional union. By correcting one's status in this way, it then will be possible to direct the de jure governments in to lawful action. As you rightly state, US citizens are at the statutory mercy of their rulers by operations of statutory law, but the expatriation act passed just before the 14th amendment provides for a path to the remedy. Eliminating international statehood for the US, inc. would leave all former us citizens essentially stateless. It is a potentially dangerous trap to seek freedom as a stateless person.
So as to the point of my original post, not that I believe this to be the right course of action, if DC became a state in union with the other 50 it would have to form a Republican form of government for its people which is guaranteed by the constitution. This of course will never happen because it is too convenient to work outside the constitution by being outside the union. The constitution only applies to the states participating in the union.
~ Engage in the war of attrition: http://pacalliance.us/redamendment/