We’re electing delegates to the convention, at least in most cases, so it’s up to who we elect as to whether they’ll cast their votes for Bush. In some cases you can try to become a delegate yourself. This year, as many of us as possible should do that, because at a brokered convention your vote will count for a lot. There’s not much else they can do. The people have their chance to decide now but we can’t have them accepting a candidate who doesn’t get a majority of the votes, because it’s possible over half of the voters don’t want him. See: Clinton, 1992 election.
You are correct. Delegates should have a second choice in mind if there is no victor on the first ballot. It doesn't necessarily have to be Jeb Bush. A second choice could well be Palin, Giulani or someone else.