It would have to be Maryland, as the land DC occupies now was all part of Maryland. Virginia got its land back in the 1800s. However, Maryland would have to agree to this, and they never would. Adding DC to the state would turn the state power structure on its head, let alone the other effects.
Right. There were many tensions between Maryland's D.C. area and other parts of the state over influence already. It's characteristic that between 1867 and 1994 the state did not elect a Governor from the D.C. area.
Precisely. It's similar to the situation in the late 50s, when admission of Alaska and Hawaii was discussed. Republicans were reluctant to let Alaska in, because it would become a Democratic stronghold, while Democrats feared Hawaii will be a Republican one. Though partisan identification essentially flipped since then, there was "balance" preserved.
Now there's no other potential Republican state to balance admission of D.C., yet alone Puerto Rico.