"Why do you believe DC statehood to be an impossibility if Democrats get the trifecta in Congress and the Presidency?"
I do remember the debate in the first Clinton term, and I think the political situation has changed so much that Democrats would jump at the chance to add D.C. to their electoral family. Back when you had seemingly permanent Democrat majorities in the House, and both chambers included many Democrats from districts with little in common with D.C., it's not surprising that they didn't want to go to the mat for the District.
Admitting D.C. as a state wouldn't require an amendment, and once that was done, who's going to stand up to defend the anomaly of the 3 extra EVs?
Your idea of New Columbia is a neat thought experiment, but I don't think it has any potential as a serious compromise, no offense. D.C. would rather continue to fight for statehood; they aren't about to give up the goal of self-government in order to be 15% of the population of a new state. Maryland won't do it and neither will any of the jurisdictions in either state, which have nothing more in common with D.C. than a federal preference for Democrats.
Didn't need to correct me on the 3 electors ("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."). I really don't think it would a problem to pass the amendment.
As to whether the Democrats would want to retrocede, on the one hand, this wasn't much of an issue in the past, so things could change. But, Maryland doesn't want DC, because it's a financial sink hole, and might have to be offered an inducement. And, as far as Democrats are concerned, if they can get an extra Congress-entity out of DC without retrocession, why would they have to bother with retrocession? They already have the 2 Senators who would be involved. The Democrats have to get it into their heads that we're not giving them a Congress-entity without accepting the responsibility of self-government, then they might take the offer.