You apparently haven't been paying attention. One thing Cruz has been very good about is getting his supporters selected to be delegates, even in states where Trump won all of the delegates. So while they will have to vote for Trump on the first ballot, those delegates will then be free to "switch sides" and vote for Cruz on the 2nd ballot - and also can vote on the rules, delegate challenges and other convention business as Cruz supporters. It is conservatively estimated that there could be as many as a couple of hundred pledged Trump delegates spots that will be filled with Cruz supporters.
That is why so many Trump supporters have been whining about Cruz "stealing" delegates. That is foolishness, of course - as long as they vote for Trump on as many ballots as they are bound, they are following the rules and nothing is being stolen from Trump. Trump has just as much of an opportunity to get his supporters into those delegate spots. He just didn't bother to learn the rules and the process of selecting the delegates on a state-by-state basis, and is thus getting outworked and outorganized by the Cruz campaign.
You are absolutely correct regarding Cruz following the rules, and I will support whoever wins according to the rules as they are currently written. Trump as a businessman played by the rules when some of his businesses lost money and declared bankruptcy - and his (or precisely his companies’) investors lost a lot of money. As a politician there are rules, he just hasn’t been as good as Cruz learning what they are and appears childish when complaining about whatever Cruz does as Cruz expertly plays the long game.
Now if the “party” finds a way to change the rules (which technically the incoming delegates can do), and someone other than Trump or Cruz is selected, the party is over.