Republicans are already showing weakness, including McConnell.
Tellingly, McConnell chose his words carefully by saying the Senate "should not" vote, rather than saying "will not" vote, leaving the door open.
Not that I wish to defend McConnell, and I don't know the context, but just going off your quotation: "will not" may simply refer to the fact that McConnell can't predict what will happen. It's not as if he's 100% in control of the Senate. It's like if I said "Christians should not sin," but it doesn't necessarily mean Christians "will not" sin. For example, not that I expect the Senate Judiciary Committee to hold hearings, but technically speaking can't the Senate Judiciary Committee hold hearings even if McConnell disapproves?
I don't know the answer to your question. My implicature is that McConnell chose not to be a leader by what he is not saying. The Republicans will cave, as they always do. Actually that is giving them too much credit. They _want_ to cave. They want to please Obama.
It's true that McConnell can't be trusted. We shouldn't merely take him at his word. However, if he backpedals, he runs a grave risk of losing his position as Majority Leader. That's a position he's coveted for years, and only recently attained. At his age, he doesn't have a second act. He was originally in competition for that position, then Bill Frist cut in line, then the Republicans lost control of the Senate. So it's been a long wait. I wouldn't expect him to jeopardize his leadership position. Boehner was dethroned for far less.
Be careful not to underestimate McConnell's incompetence and perfidity, nor his perverse willingness to gladly pay for the opportunity to politically prostitute himself.