That's not quite how it works. If the election is thrown to the house, the vote is by state, with all the representatives for each state voting to determine how their state's vote will be cast. This means that the number of Republicans and Democrats isn't the determinative factor, but the number of each in each state's representation.
And that's where 2010 was the ace in the hole. Because of gerrymandering following the 2010 statehouse routs, the GOP was able to squeeze out more Republican districts in each state. That's also bad news for Trump, because in the primaries, his strongest states were blue states in the Northeast that will almost certainly vote Clinton in that scenario.
Combine that with McMullin's inside connections as a policy hack for the House GOP, and the odds of a President McMullin look a lot better. Perhaps that's why the Trump caucus is scared all of a sudden.