As much as I might like it to be the case, being a political scientist does not make me 100% informed about everything that can happen in American politics. We all have areas of expertise and beyond that the depth of what we know is not necessarily greater than any interested non-academic. Deep congressional procedure, for example, is a mystery to me. If you ask me about the Byrd Rule and the steps necessary to consider an omnibus bill under reconciliation. I have people for that, and without exception they can be taken out in public only infrequently and only at great risk.
That preemptive mea culpa is to admit that party nominating procedures are not something about which I have universal, all-encompassing knowledge. However, I've taught and worked with the topic enough to be comfortable with it. So I'm going to attempt to explain why the GOP is going to have a very, very difficult time setting up a "brokered convention" to stop Trump.
Brokered conventions are to politics what surprise witnesses are to the legal process – they're largely a narrative device used to create interesting scenarios, not a thing that actually happens in real life. They make good water cooler talk but the parties both have rules and procedures in place specifically to preclude them.
I'll keep this as basic as possible. Candidates compete in primaries, caucuses, and conventions in each state (and territories, oddly enough) to win delegates. Delegates are awarded on either a winner-take-all or a proportional basis. The early contests – the ones completed so far – must by Republican rules be proportional. The Democrats do it differently, but let's disregard that for now. Later GOP contests are mostly winner-take-all. The GOP used to do all W-T-A contests but changed recently when they realized that one candidate who did well for the first two weeks of contests would essentially wrap up the nomination. So this stretches the process out and generates more competition, because candidates who do not win the state still can win delegates under proportional rules.
Delegates are either committed (Republicans tellingly prefer the term "bound") or uncommitted ("unbound"). Committed delegates must vote on the first ballot at the national convention for the candidate that won said delegate in a primary. Uncommitted delegates can vote for whichever candidate they want. Some states award only committed delegates, and some states award only uncommitted. It's a very confusing system and all fifty states have unique rules. When a candidate wins committed delegates and then drops out of the race, those delegates become uncommitted.
What is not complicated is this: The GOP awards the nomination to whichever candidate gets a simple majority of available delegates. 50% + 1 of the current delegates is 1,237. Trump currently leads with 384 to Cruz's 300. The proportional system has treated Ted Cruz well, as you can see. Despite winning only four contests so far he is not terribly far behind Trump. If any candidate gets to 1,237 committed delegates, the race is essentially over.
There are two relevant internal rules in the GOP that must be understood here. One is a requirement that no candidate can be nominated who did not win a majority of delegates in at least eight states. That is crucial here. They can't nominate Mitt Romney at the convention no matter what. Nor, as things currently stand, could they nominate Cruz (four wins) or Rubio (one win) or anyone else (zero wins). The second important rule is that the Party can alter its own rules at the national convention without waiting until the next before any changes take effect.
So for a "brokered convention" one of two things must happen. First, the non-Trump candidates could win enough delegates to prevent Trump from reaching 1,237. If no candidate entered the convention with a majority, the first ballot (remember the committed rule) would be unlikely to produce a victor and then all bets would be off. Second, they could change any and all of the rules mentioned here, essentially blowing the whole thing up in a sort of anti-Trump Thermopylae; a last ditch, scorched Earth play. Cut off the limb and suffer in the short term to save the brand in the long term. And no, Trump could not mount an independent campaign at that point because in almost every state the deadline to get on the November ballot will have passed.
In short, to have a nominee other than Trump if and when he crosses 1,237 delegates would require the GOP to change its rules to disregard the entire primary process and let party insiders hand-pick a candidate. You can imagine how well that would go over with Trump supporters and Republicans in general. Then again, they might judge that to be worth it compared to letting Trump not only go down in flames but also to take god knows how many down-ballot candidates with him.
Finally, June 7 is likely to be a significant date, with California's 172-delegate, winner-take-all primary taking place. It's the last date for primaries and caucuses and if Trump has not secured 1,237 delegates already by then he will at least be in striking distance so that California would put him over the top.
Off the cuff it seems unlikely that the Republican Party would do anything as drastic as changing its rules at the convention to render moot everything that has happened in the nomination process to that point. It all depends on how effective the powers that be inside the party are at convincing themselves that Trump won't be so bad. The other issue is that the candidate best situated as a Trump alternative may be even more strongly loathed within the party.