BTW, it seems counterintuitive, but giving DC to Maryland would probably be a wash in the House. That's because DC isn't populous enough to comprise one House seat on its own, therefore they would have to shift about 200,000 people out of the adjoining MD-08 or MD-04 districts (or both). This would almost certainly not leave enough Democrats in the rest of the state to make up six hard left districts. At the very least you'd quite likely end up with a pure toss up district in addition the two solid GOP seats.
Concerning the possibility that DC would be retroceded (the way Arlington was, a long time ago). DC had 572,000 people in the 2000 census (but is now down a bit). This is a bit below the average for U.S. Congressional seats (647,000). Maryland, has a bit above the national average in its Congressional districts (on average 663,000), so that it would make sense to simply retrocede DC to Maryland and award Maryland one more seat, increasing the size of the House to 436 seats. The state of Maryland would need to re-draw its lines (state legislature as well as Congressional), so as to move something like 100,000 to the district (if that was what they wanted to do). I suppose they'd try to keep the black opportunity district involving Prince Georges County intact (this is to the north and east of DC), so they would get the additional voters from Montgomery County (to the north and west of DC), and then shift other lines here and there. Possibly, this would make the district north and east of Baltimore, formerly represented by Bob Erlich and now represented by a Democrat, more competitive; or maybe the district including Anne Arundel County formerly represented by Ben Cardin; or maybe the district in "southern Maryland" represented by Steny Hoyer. But, I doubt much of anything would change in any of the Maryland districts except that lines of the district mainly involving Montgomery County would be shifted a bit. Retrocession would not require a Constitutional Amendment (it didn't with Arlington), just a pact between the federal government and the state of Maryland. Any reasonable pact would fly through the Congress and the state of Maryland (both being controlled by the Democrats), and get the President's signature. On the other hand, eliminating the special 3 electors for the district would require a Constitutional Amendment, so that the pact might be made conditional on that. The concept of a federal district, while interesting at the time of the founding, is just not as important as the principles of "no taxation without representation," and of "one man, one vote."