Also, there is no candidate who will alienate more voters in a brokered convention than Kasich. It is one thing to bring in someone who wasn't running but another to nominate the one who everyone voted against.
Opposite. I don't think many people who voted for Trump/Cruz did so because they hated Kasich. I do think many people that voted against Trump/Cruz did so because they hated Trump/Cruz. Both of them divide the republican party, for better or for worse.
You are correct that a brokered convention runs a serious risk of fragmenting the republican party further, but that risk exists if any of the 3 current candidates are selected. Again, the exit poll results for the states that have had primaries so far, especially the swing states, are frightening.
I am sure the party leaders and delegates will try, as best as possible, to use the convention to unite the party in favor of the candidate they select and against Hillary. You are correct that this will be difficult to do.