The fireworks will be at pre-convention and convention, regardless of how many delegates Trump collects. There will not be a third party that will emerge that will threaten both Democrat and Republican votes to where the delegates will have a veritable split.
Most likely to happen would be the GOP outright deny Trump the nomination through drastically re-working the nomination rules and brokering the convention. This is what they should do, because of the campaign Trump has run.
This will cause outrage within the party and it will damage the party. However, the alienation will be from the far-right fringe that has chased Moderate Republicans out of the party to begin with. In the long run, it will be good for the party because it shouldn’t be run by the far right.
Second most likely would be they bow down and tuck their tail between their legs and get behind Trump, and the #NeverTrump movement votes for a third party. The Libertarian Party is now on 30 ballots for the election, and working on securing all 50 ballots.
59% of Republican voters who did not vote for Trump in yesterday’s staw poll stated they’d strongly consider a third party candidate. Even if a third of those people actually stick to their word, that’s enough to take away from Trump and give the election to the Democrats.
Third most likely (kind of in tandem with 2nd) is that #NeverTrump will vote for Hillary instead, because Trump is that much of a danger. Whatever happens, a Trump nomination will give the election to Democrats, and the Republican party will be hugely damaged.
The GOP is in kind of a lose/lose situation, and their best chance of running a campaign that leads to a victory is to take control of the delegates in convention and nominate the candidate that has the best chance of winning against Hillary in the election.
Leaving Trump in has the potential to alienate 70% of the party, versus 30% of Trump supporters (if even that). The math there seems pretty simple, so I can’t imagine they won’t run the risk of alienating the smaller margin as opposed to the larger margin (as was done in 2012).