There was a lawsuit against the Republican party a few cycles ago about some issue with delegates or getting delegates. The courts have ruled political parties are private and this can make any rules they want.
He is not stealing delegates he is earning then. Cruz supporters show up at conventions and get elected to be delegates. If delegates are important to the Trump campaign, Trumps supporters should do the same thing.
Not going to happen. If Trump wins, he will have to go through Cruz to see who the VP will be. More than half the delegates at the convention will be Cruz supporters. They will determine who the VP will be.