"How much of a shift would it take to first guarantee a Trump win, second for it to be a landslide, and third, for it to be a sweep?"
Well, as I pointed out below, DC is the biggest reason why there will never be a Republican electoral college sweep.
In 2016, 90.86 percent of all DC voters picked Hillary. In none of the 8 wards did she get less than 80% votes cast, in 5 of them she got over 90% of the votes cast.
I mean hell, given the political situation in the country in 1984 and and Mondale still got over 85% of votes cast in DC.
I'm not sure, but I don't think that a Republican presidential candidate has ever won a single DC ward since DC residents started voting in presidential elections in 1964.
Ah, here we go...
From the Wikipedia entry on DC results in the 1980 presidential election...
"Carter's 74.9% of the vote represents the lowest vote won by a Democrat in the District of Columbia."