Good post.
As to your questions, it is probably all of the above, plus an unmitigated hatred for Pres. Trump, his supporters (see my tagline) and everything standing in the way of full-blown elitist-ruled socialism.
IMO Trump would rather face Warren or Sanders rather than Biden, despite his obvious failings. Trump wants 2020 to be about socialism versus capitalism. Medicare for all, the green new deal, open borders, amnesty, etc. will make it easier for Trump to campaign on the issues. It is Reagan versus George McGovern. And blacks will not turn out for Warren or Sanders like they would for Biden. There will be a minority on the Dem ticket, but as a VP candidate, it will appear to be more token than anything else.