You don’t understand conventions. Candidates attempting to sway the delegates is how they have always operated. Unless the law in your state binds the delegates to vote a certain way, they are free agents, free to be lobbied by anyone. Both you and your candidate have the right to lobby the delegates or support people to run as delegates.
Have we always had elected GOP leadership meeting in secret with rich DEMOCRATS? Of course not. You presume this has been a fair fight and it has not been.
So say Cruz steals the nom from Trump. Then Trump pushes the GOP destruction button on his phone. What has Cruz gained? Nothing. Except a dogfight in 2018 to keep his Senate seat. The bottom line is Ted Cruz is never going to be POTUS and the sooner Cruzers realize that the better for everyone.
Those are lots of wild predictions, but what the polls show is that a huge number of Republicans are considering not voting for Trump if he’s the nominee. At this point, that outcome is more realistic than your more fanciful one.
In theory, I believe Trump could improve his numbers and support if he became a very solid, disciplined, gaffe-free candidate for the next 7 months. But I also don’t believe people can change who they are. And Trump appears defiant and resistant to any criticism, constructive or otherwise.