Oh, I have no doubt the left will throw everything they can at him.Maybe you're right but it's a long way until November and I don't think we've even begun to see some of the crazy stuff that will likely happen between now and then. Lawsuits, convictions, Newsom, Obama....who knows.
But one advantage Trump has is that most of it is likely to have no effect. They've already tried every trick in the book and wrote some new ones; no one will be surprised at much else they try. At this point, people just roll their eyes at new dirty tricks.
As for substitute candidates...Manchin is the one that concerns me. If the Dems can bring themselves to run him, he would likely win.