How would there be any pressure for them to vote Hillary? Romney won 24 states with only 206 EVs. If Trump is close enough for it to go to the House, he will have over 25 states necessarily. If any one of those states votes against Trump, they’re defying the will of their own voters. All the pressure is on them to vote for Trump. Only a minority of states will have pressure to go Hillary.
This is a function of the fact that there are lots of low population red states, while the Dems win by winning more of the high population blue states.
They won't have to vote for Clinton. If Hillary wins more popular and more electoral votes, you'll see GOP reps in some states going for McMullin to split the Republican vote so Hillary wins the count in the House.
Probably that won't happen in many states. Maybe Pennsylvania, Arizona, Colorado -- if Trump carries those states, and the race goes before Congress, he may lose some votes there to McMullin.
I also notice that some states that Clinton may carry have Republican majorities in the House -- Michigan, Wisconsin. What those reps will do is also something to speculate about. And what about states whose delegations are equally split between parties, as New Hampshire's is now? They may not even figure in the voting.
No way do I see McMullin becoming president. Washington doesn't hate Hillary enough -- or even fear Trump enough -- to give the White House to somebody who hardly anybody voted for. But DC may fear Trump enough to see to it that Clinton gets the prize (without necessarily voting for Hillary themselves).