Gee, let's see. Mitt just spent five years and millions of dollars - including a fair chunk of his own coin - winnning the nomination. Do you seriously think he would, barring an incredible scandal or serious illness, walk away from it now?
I think not.
I think that if there could be a floor struggle at the convention Romney has the best chance of winning. However, a floor fight would be a hard push on Romney to keep a conservative platform. If Romney would win the nomination and then POTUSA ,I believe he would be very constrained to stay with the platform or face only one term.