You raise an interesting question. As a pure hypothetical, I would love to see an open convention, for reasons that have nothing to do with this year. When's the last time we had one? I think our politics were healthier back in the day. Let's go 102 ballots and see what happens.
I don't think it's at all likely, but if it does happen, what would an unbound Convention do? Suppose a clear majority of the delegates made it clear that they would NOT nominate Trump. (A majority of Republicans continue to vote for someone else, and a lot of them by now are for anybody but Trump.) One possibility, probably the likelihood, is that Trump would stomp off and run as an independent. The other is that the Trumpsters, once they toweled off from peeing themselves, would sit down and deal. Things would be pretty fractious. Could anyone unify the party? Assuming that Trump would come in with a plurality and by far the angriest group of supporters, he could probably name the unity candidate if he is willing to play ball. So the question becomes, can Trump put the country ahead of himself, who would he support as a consensus choice, and what conditions would be attached.
The choice could even be someone who was not in this year's race, or who got out early. (And no, I don't mean George Pataki.) Is there a governor or senator of unifying appeal who is unmarked by this year's infighting. Or even a major figure from outside politics? I can't think of a non-political figure who might do the trick, but that is my inadequacy; I'm sure there must be someone out there.
Conventions used to be like this routinely. It would be fascinating to do it again. We might even get away with it politically. It would be a riveting and highly dramatic story for the news media, and whoever emerged will presumably be running against America's least-trusted public figure, someone who the public knows should be under indictment, etc.
The reality is that the primary process is almost certain to produce a winner, so the above is a parlor game. But it's fun to think about. I've never been convinced that the modern primary system is necessarily the best way to choose a nominee.
I think 1948 was the last GOP ‘open’ convention.
The last winning nominee from an open convention was Franklin Roosevelt.
There will be no open convention. If a candidate doesn’t have enough delegates, that will be known long before the convention opens. The convention rules committee would then propose appropriate changes (subject to whole convention approval) to assure a first ballot winner.
Think about the logistics of an extended convention. Hotel and travel chaos, media coverage chaos, livelihoods put on hold, etc. Any deals will be done before the first ballot for the nominee.