Mitt Romney and Donald Trump couldn't be more different. It seems bizarre that you're trying to draw any conclusions by using Romney's numbers to predict Trump's.
Mitt and Trump couldn't be more different? Really?
Compared to who? Obama? Hillary?
It's not a comparison of just candidates. It's a comparison of choices of candidates.
The choice in '12 was Obama vs Mitt. The choice in '16 would be Hillary vs. Trump.
The number of voters who voted for Obama instead of Mitt in '12 and who would switch over to vote for Trump instead of Hillary would be small.
It's as I said.