Here's the letter from 39 law professors saying that D.C. statehood is perfectly constitutional and in practical terms acceptable.
Here's my additional thoughts.
Congress has the right to make any qualifying territory a state that it wants. That it would happen to give the Democrats two more senators is just a good fortune they're taking advantage of. It's been done before. Collectors of the state quarters may have noticed the cluster of six northwestern states that were admitted in 1889-90. It would have been five, except that Dakota Territory was split into two. The reason for that is the Republicans had just reclaimed control of the Congress and Presidency, and wanted to take advantage of these Republican-leaning territories to improve their prospects. And, indeed, 12 Republican senators from these states did change the party balance of the Senate in some upcoming years.
The Federal District isn't necessary. Article 1 Section 8 of the Constitution gives Congress exclusive power "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." Notice first not exceeding: it can be smaller than that. Notice also as may. It does not read to me as if we're required to have such a district.
Needn't the District go back to Maryland if dissolved? No. When the Virginia portion was retro-ceded in 1846, that was voluntary on both the federal and state parts. Both Virginia and Maryland gave up all rights on ceding. D.C. could go back to Maryland, but Maryland doesn't want it and the inhabitants of D.C. don't want to go there, so in practice that's out.
What about the 23rd Amendment? The 23rd Amendment is what gives D.C., though not a state, electoral votes for President. The (legally: see below) uninhabited rump federal district would retain this right when the rest of D.C. attained the vote by right as a state. What then? Well, as the letter points out, we could repeal the 23rd, the way we repealed Prohibition. In the meantime, the letter says that Congress could just revoke the enacting legislation, because Section 2 of the 23rd gives Congress the "power to enforce this article by appropriate legislation." True, it does, but it seems to me that Section 1 requires Congress to enact such legislation. For it says, "The District ... shall appoint in such manner as the Congress may direct: a number of electors ..." Shall appoint. That seems to say there must be such electors. But also in such manner as the Congress may direct, which means Congress may direct the choosing of electors required to vote for the national electoral vote winner or popular vote winner or whatever.
Wouldn't the President and his family, who live in the White House, be voters in the rump District? Not unless they chose to be. They never have in the past. Members of Congress, though in practice living in or around D.C. due to the length of Congressional sessions, are representatives of their states and keep their voting residences there, and Presidents do the same thing, partly so that they can be photographed at polling places. The Obamas voted in Chicago; W. in Texas. DT was registered at Trump Tower until he moved it to Mar a Lago.
There is no city in D.C. called Washington. There used to be; but when the city was expanded to be coterminous with the district, the city government was dissolved. When home rule was granted, it was still under the name of the district. The mayor's official title is Mayor of the District of Columbia, not of Washington. The current statehood bill would make it the "State of Washington, Douglass Commonwealth." The rebranding of the "D.C." part is brilliant, but including "Washington" is unnecessary and only would increase confusion with the other state of Washington, which is bad enough already.