At this point, I'm convinced there are only two ways the Republicans turn on Trump. Either Trump goes full scorched earth and labels the GOP congress as the enemy, basically imploding the party, or it becomes incontrovertible that Trump colluded with Russia to his own benefit in the election. There's practically no other way in my mind, barring some kind of psychotic break by Trump, that the party turns on him.
That's not how this works. At most, you'll get President Paul Ryan until 2020.