If Trump want to be the nominee, he must get 1237 delegates outright. I think the convention won't be kind to him. If all the delegates remain loyal, then there will be 50 votes with no candidate. A deal will have to be made. Bringing in an establishment pick will not work. It will be Trump or Cruz. If Trump keeps looking weak, the delegates will switch to Cruz.
So the threshold of 1237 only applies to Trump. Everyone else still stands a chance without it except the Frontrunner. Brilliant plan!
Many of the delegates will come in already favoring Cruz. They’ll be bound on the first ballot or two to Trump, but as soon as they are released, they can switch. There can also be “faithless electors” who decide to buck the rules and vote for whoever they want, despite their state’s rules. It’s unclear what the mechanism would be to enforce the binding on those delegates.