Would Trump have won if he had stared a 3rd party and not campaigned to be the GOP nominee? The answer is no, and I think that is true for anyone who follows that path. In fact, Trump brought more people to the Republican party that any other candidate in history. The GOP is their party, and they should fight for it in the primaries.
The GOP despises Trump. They are every bit as happy about his departure as the Dems are.
Those who still harbor hopes for the GOP can remain in the party. But its a sinking ship that has rotten and decayed over many decades. Its time for the Grand OLD Party to be scrapped.