If McConnell really wanted Trump to win, he would have promised to keep the seat open until 1/20/21 and I honestly think that would have been the end of the Biden candidacy. Every single one of the Republican voters for Biden would have come home immediately and it would have been over. But he's not going to do that because there's still a pretty big chance Trump will lose, so he's taking the safe route of ramming through a vote right now.
Not at all. Mitch promising to keep the seat vacant wouldn't have outweighed the many other reasons that people are switching to Biden.