Supporting Republicans down-ballot isn’t in conflict with voting 3rd - party for the Presidency.
I’m not saying I would support a third party. But if Mitt wound up the nominee - especially if he got it by Democrats voting for him in open primaries, which is the most likely scenario - frustration will likely boil over.
That’s my reading of the situation. YMMV. My predictions of the future don’t always come true.
Another thought. If Mitt wins the nomination, which of the current candidats do you think would agre to run as a third party candidate?