I don't think the Tory rebels necessarily want to get rid of May. They want leverage to control her and get her to renegotiate bad parts of the draft deal, in particular the fact that the EU will have power to bind Parliament's hands and keep the UK in the Customs Union in perpetuity. If a vote of no confidence is a real prospect, May will face this pressure from outside, and she now has a cabal of 5 ministers (Gove, Leadsom, Fox, Mordaunt and Grayling) that will bring pressure from within the Cabinet.
May was deluded in bringing the 2017 general election. She is deluded if she thinks she can get this bad draft deal through Parliament. If she can be persuaded to make necessary changes, I think, faced with no deal, the EU would come back to the table on specific sticking points.
If she won't bend, I think the Rebels may put their weight behind Raab if he is agreeable.