To exercise exclusive Legislation in all Cases whatsoever, over such District (not exceeding ten Miles square) as may, by Cession of particular States, and the acceptance of Congress, become the Seat of the Government of the United States, and to exercise like Authority over all Places purchased by the Consent of the Legislature of the State in which the Same shall be, for the Erection of Forts, Magazines, Arsenals, dock-Yards, and other needful Buildings; And
The "Seat of the Government of the United States", is as clearly defined in Article 1, as is the powers of the Congress themselves.
To make it a state, the Congress would have to cede it's authority to the new State Legislature, something that SCOTUS rejection of the Line Item Veto expressly forbade.
One would think that the state that ceded the land might want it back if it is no longer being used for the purpose for which it was ceded.
honestly, my brain is fried and i am incapable of coherant thought. i wrote there what he sent me in an email. i consider him to be the person i know best who knows the most about the Constitution, but i will ask him to explain and let you know.
not if they formed an enclave of federal buildings containing all of that, and annexed it as DC, making the residential areas a state. it is debatable, but it looks like it could be done with passage of a statehood bill and not a constitutional amdt. Nowhere is it written that it is absolutely required that there be a constituational amdt for statehood. Hawaii and Alaska did not require it, so far as i can tell, and maintaining a federal enclave known as DC, could get around the constitutional provision you referenced.