I wouldn't say that it's guaranteed that Trump would be elected if the election was thrown into the House. You'd have enough Republicans who refuse to back him that things could get interesting, and McMullin could matter greatly.
Refusing to elect a GOP Presidential candidate would destroy the party. There's no way House Republicans would be that stupid.
Not if they manage to hold the Senate, and can thus get President Pence instead. And it's not like it would necessarily take that many defections to block Trump in the House anyway. The House Republican caucus would mostly vote for Trump, but a few defections could sink him.
Couping the Trump faction wouldn't start a civil war because they would accept President Pence? There is no scenario where ~50% of the country would vote for a candidate as President and then his party would accept him being couped. The long-term irreparable damage done to the GOP would outweigh the costs of simply letting Trump into the White House and containing him institutionally.
- If the House simply lets the position of President go unfilled, the Vice President (so, Mike Pence) would then become Acting President. Note that the House must vote on the position of President "immediately"; this probably means that they can no longer elect a President after they go on their first recess after the election and the Presidency is recognized as vacant, so the Vice President accedes to the post by default. Pence would become President in this scenario even if he actively opposes it and lobbies House members to elect Trump.
In that scenario, couldn't Pence appoint Trump as VP and then resign, defeating any intransigent NeverTrumpers in the House? Even if there were enough of them in the House to suicide the party?