If that's not what you meant, OK, it's just that "As much as many of us -- including myself -- tried to fool ourselves into thinking a majority of the country wouldn't elect Donald Trump as president" sounds like you were saying reality was pointing to Donald winning and those of us who didn't think he would were grasping to find any evidence he wouldn't, when it's closer to the other way around. I think I get it, though, you were talking about the left hoping people were considering Donald a total nonstarter.
Undecided voters could be undecided for any number of reasons. I don't know why party control of the White House has to be the reason. If it's just people wanting new leadership, I don't think people suddenly start feeling that way a week or two before the election, unless there was some big failure of the Obama administration just before the election. Seems like the news of a candidate being back under FBI investigation would be a much bigger factor. I don't know if there were any polls conducted actually asking undecideds what exactly changed their mind, though.