In the amazingly unlikely event McMullin resulted in neither Trump nor Clinton getting to 270 the House would pick Trump. Those guys jobs are on the line every two years. There is a big majority of House delegations that are GOP - Each state gets one vote. A few could be flakey and Trump still wins. Evan’s only purpose is throw the election in battleground states with Mormon populations.
And the only state where Evan matters is in Nevada, but it’s a big one. Any state where McCain and Romney won is not likely to be turned as a result of Evan. A lot of these states the Democrat has trouble even getting a third of the vote, so it’s mathematically impossible for a 3rd party candidate to throw it to the Dems.
Nevada is the big problem though. Trump almost certainly NEEDS to win Nevada. But Obama won it twice, by 7% last time. Bush won it with less than 5% margin. It’s the 6th most Mormon state, with them at 4% of the population, but they’re likely to be an even higher percentage of the voters.
Trump needs every vote he can get there. That is the one state that Evan could easily throw to Hillary and it is a crucial one.