I. Trump gets the nomination
II. Trump doesn't and instead
A. Fades away and/or decides to play nice (that second one just got even less likely)
B. Runs as an independent
C. Causes as much trouble as possible
Unfortunately, while I've been procrastinating, Chait has been writing, producing a string of really sharp posts that are on track to anticipate all of the major points I wanted to make. He already gave us the definitive brief analysis of Scenario I and today he covers most of what I wanted to say about II.B.
Chait is the only news analyst I've seen so far who is seriously thinking through the GOP's dilemma: they can't afford to keep Trump but they can't afford to lose his supporters. The Republican model requires very high turn-out from the base. Pissing off a large segment of those voters could have significant consequences.
Chait actually goes so far as to suggest that the party's best way out of this dilemma might be the much-feared third party run:
2. A Trump independent candidacy would have down-ballot benefits for the party. Trump would split apart the Republican vote at the presidential level, but the socially conservative white working-class voters who turn out to vote for him would overwhelmingly pull the lever for Republicans in Congress (and in state elections). The deepest risk Republicans face is the prospect of an electoral wipeout that puts its control of Congress at risk. An independent Trump candidacy would close off such a prospect.