Romney is an unlikable tool who was the conservative in '08, a 'severely conservative' RINO in the '12 primaries, and literally stood for nothing in the GE. Additionally, Trump is not a gaffe machine. He knows the effects of everything he does when he does it. Romney wasn't coming out in support of government-run health care and such. He was an out-of-touch elitist who didn't care about the working man. Trump has the opposite base.
Hah. You give him too much credit. He has made plenty of gaffes, but he has gaffe-armor as thick as a tank. You shouldn't confuse his supporters not caring one bit with him having a flawless, expert strategy (though, in my opinion, his strategy has been expert for a primary election, but he has made plenty of mistakes and gaffes executing said strategy)
He'll try to weasle to the center and then claim he did it to show how bad other candidates could have been and then make an excuse for why he actually is a moderate/liberal
I'm skeptical that general election voters will be as receptive to his flip-floppery as the Republican base, but anything can happen.