Mitch’s apparent willingness to move forward may indicate that he has some degree of confidence in the vote. I could be wrong about that, but its hard to see how he would commit to doing it in the absence of having made some prior political arrangements.
Because he knows what failing to support this will do to Republicans and the republican party. Sure they all want to take out Trump, but it's a bit like trying to throw a 1 megaton nuclear handgrenade. You will take out the target, yourself and a whole lot more.