I can’t imagine that she would skip this debate. That would just continue to lend truth to rumors that she is not well. It would dog her campaign and drive her poll numbers down further. DNC officials would start lobbying her to drop out fearful that there is truth to the rumors that she’s not well.
However, that being said Trump has to prepare for this possibility however remote. If that were to happen, I think he should go ahead with the debate. However, it should just be him against Lester Holt. Don’t allow Johnson or Stein on the stage. That would give them stature they have not earned. I wouldn’t mind Stein because she would only draw lefty loonies away from Clinton, but you would have to let Johnson on also (since he has higher numbers than Stein) and he could possibly pull votes that might vote for Trump. Why take that chance? He hasn’t earned the right to be on that stage. He could have ran in the Republican primaries if he wanted.
He could turn it into a 90 minute press conference. Let them all have at him.