Get this straight.
Donald Trump is not winning a majority of the GOP primary/caucus votes. He's getting only about one-third of the vote. Therefore the GOP convention can readily deny him the nomination for the solid reason he lacks a majority of support among the party's voters.
If the present GOP rule says the nomination must go to the one with even just a plurality of delegates, the convention can simply change the rule to require a majority of votes. That's quite democratic (pardon the small-letter "d" word, you GOP friends). After all, why should about one-third of the Republicans who voted in the primaries and caucuses determine the party nominee when twice as many GOP voters rejected him?
As for the winner-take-all rule that kicks in soon for GOP primaries, even if this gives Trump 100% of the delegates in a state, the convention can change them post facto to be proportional to the vote results in the states.
Can a convention do these sorts of things, change rules in mid-stream or even at the last moment?
Of course it can. Conventions can make their own rules and at any time they want to.
I witnessed such in 1980 as a Kennedy delegate to the Democratic National convention. Jimmy Carter was seeking re-relection but struggling with just getting the nomination. Why? Because he was at 22% popularity by the time the convention opened. The delegates who had been chosen in primaries as his supporters were willing to switch from the sure-loser Carter. So the Carter establishment Democrats bulldozed through a new rule forbidding delegates from changing to another choice. They did it by extortion, promising the "Carter" delegates to make their lives miserable for the rest of the convention and after they got home. They also attempted to bushwhack Kennedy instigators like Dolores Huerta, cofounder with Cesar Chavez of the United Farm Workers, and me. She and I together narrowly eluded some rough stuff while organizing the attempt to switch delegates away from Carter. (When President Obama gave her the Presidential Medal of Honor 30 years later, he couldn't have known about her involvement in that strange moment in 1980. Ah, Dolo, those were the days my friend!)
So everyone should calm down. It ain't going to be Trump.
Oh, oh! I got that wrong! Don't calm down, everybody! If it ain't gonna be Trump, it's gonna be Ted Cruz!
So go ahead with sweating it, you GOP friends. With Ted Cruz as your nominee you are still waist-deep in the Big Muddy.
Unless Hillary Clinton is the nominee. She's currently losing to Cruz in head to-head matchups.
So it's still a wild year.
Can we be saved from the perdition of a Cruz presidency by a 74-year-old socialist Jew who looks like an unmade bed?
As the United Farm Workers' motto says, "Si, se puede!" Yes, we can!
P.S. Dolores Huerta is now supporting Hillary Clinton. Dolores got old. I never did. But I still love her. You can't work side by side with someone doing campaign mailings until two a.m. in a county supervisors race and also brave the bad guys at a national convention and not still be friends no matter how mistaken that other person has become. I forgive you, Dolo. Vaya con Dios!