The only real problem with your scenario is that Cruz will be lucky to get 30% of the remaining delegates. Trump will get more than the 1237.
So when Trump shows up at the convention over the mark, and the GOP changes the rules, then the party is over. Literally.
I think that if Trump hits 1237 and then spends weeks or months in the media as “the presumptive nominee” it’s going to be beyond the pale, even for the GOPe to take the nomination from him at the convention. I don’t think they’ll try.