If no one meets the standard for a first ballot win, they start trying to change the delegates minds on whom to vote for.
They can buy votes, intimidate, or any other thing they can come up with.
They can try to make a deal with the candidate like with Reagan by insisting he name GHW his running mate.
At some point they can enter another name for consideration including someone who has not even run in the priamary (Mitt or Daniels).
If Trump wins at the convention, I am almost sure they will run Mitt or someone else as an independent 3rd party. The same thing happened against Reagan in 1980 with John Anderson, a RINO Republican.
I’m not so sure they would do the same thing to Cruz. They’d have a much tougher job saying Cruz was totally unacceptable, because his tone is so measured and he doesn’t make outlandish or profane statements. Cruz’ negatives simply aren’t as high as Trump’s, so the idea of a lot of the public saying he’s “unacceptable” is not plausible.