If Trump falls short of 51% they have every right to open the convention.
But if they do so, just about all of his voters will sit home in November. They have to ask themselves if they really want to do that.
No, if he falls short they HAVE NO OTHER OPTION than to open the convention.
You didn't answer the question...What should be done?
If Rubio picks a 78 year old Catholic grandmother from Deluth Minnesota name Vera Tranelli as one of his delegates, she is bound to him for the first ballot. How do you propose she should be bound to Trump for the second ballot? Even IF Rubio released his delegates and asked them to vote for Trump doesn't mean Vera would. Maybe she hates Trump. Maybe abortion is her #1 issue and she wants to vote for Cruz.
How do you propose we change the rules so that Vera is FORCED to vote for your candidate on the second ballot?