Why would the race be pushed to the U.S. Congress?
If no one wins a majority (270 or more) of the 538 electors of the electoral college, the election goes to the House of Representatives. This is important to Evan McMullin since he doesn't have a real path to 270 electoral votes. He may win Utah, but it is unlikely that he'd win anywhere else. The only way he could get to the presidency would be if there was an electoral college deadlock followed by a vote in the House of Representatives.
If the election goes to the House, the Republicans will have enough state delegations to carry the vote. The general thought is that there are Never Trump Republicans who might combine with Democrats to elect a candidate (e.g. McMullin). But if the House deadlocks (no candidate wins 26 or more state delegations), the presidency goes to the Vice-President selected by the Senate.
If the Democrats win the Senate, they'd vote for Tim Kaine as VP. If the Republicans win, they'd vote for Mike Pence. If no one wins, it would either pass to Paul Ryan (if the normal line of succession applies and he's still Speaker) or Congress would have to pass a law covering that situation. Or Angus King might switch and vote with the Republicans (he's an independent who caucuses with the Democrats). There's also some speculation that Biden might be able to break a tie in that situation since the vote occurs while he is still in office.
Republicans would prefer Pence or Ryan to McMullin. Democrats would prefer Kaine. For Democrats, he's just a less experienced version of Pence or Ryan. Republicans might prefer McMullin to Kaine but then it's a question if there are enough Republicans willing to compromise on McMullin over Trump and Kaine. Pro-Trump Republicans could reasonably point out that Trump would have considerably more votes than McMullin, and they aren't known for their ability to make pragmatic compromises.
So even if McMullin is successful at winning Utah, it's not evident that he has a real path to victory. It's more likely that that creates a President Pence or Ryan. Or even Kaine.
And as @origimbo already noted, neither Nathan Johnson nor Mindy Finn would be likely to qualify for the Senate's Vice-President vote.