So a real compromise candidate might be possible. It'd likely depend on Obama and the senate Democrats as well, though.
it's actually pretty unlikely. Fivethirtyeight has a bunch of good articles about this, but the simplest piece of information pointing to it: Trump is intensely disliked by about 60% of the voting public. He's got a vocal and enthusiastic following, but once you go outside that bubble, he simply doesn't have room to grow.
I voted for Bush twice, McCain, and Romney, and I'll vote for either Hillary or Bernie (two candidates I can't stand) over Trump, and I'm far from alone.
Bush and Cruz are in my "hold my nose and vote" tier.
Carson isn't presidential material IMO.
That's bad, because it will perpetuate party politics. No matter how much people dislike the candidate nominated by their party, they'll think they've got no recourse but to vote for him/her lest they lose the Court.
All the Republicans I know who say they'll vote for Clinton before they'd vote for Trump - and there are plenty of those - won't follow through. The chance we might have had for an olive branch, reaching out of the viper pit of party politics, is now gone.