Quite a few folks that I respect are in the #Nevertrump universe, and I share the gut feeling that it flows from. However, I'm not quite there, at least for the general election. If minimizing the damage from a bad choice is the goal, he might be enough less worse than Hillary to have me refrain from a protest vote..might.
For the primary, I'm definitely there; he's ill-suited for the job.
Trump is on pace to come close to a majority of delegates, depending on how the last batch of primaries fare. However, a plurality of delegates isn't enough, unless you want to change the rules to enable that.
The guy with a plurality of delegates will have the inside track to win over enough delegates from other candidates and patch together a majority, but if a majority really doesn't like Mr Plurality, he won't win. That often happens in parliamentary systems where the party with the most seats can be on the outside looking in if everyone else hates the plurality party. Recently demoted Andrew Jackson had that happen to him in the 1824 election, where he got the most electoral votes but the number two and three candidates (Adams and Clay) pooled their support in the House to get JQ Adams elected. Jackson was steaming for four years and won the rematch in 1828.
Spain is dealing with that right now, where their main conservative party came in first, but can't get any of the centrist and leftist opposition parties to be junior partners. Things are so contentious that no working majority was come to and a second election is the likely result.
Things are about as contentious in the GOP. In Trumpese, Cruz's first name is Lyin'; well, Ted is his middle name on his birth certificate (Edward to be fully accurate). It's hard to see Trump taking no graciously or Cruz bowing out in the name of a unity ticket.
We're in very uncharted territory with some interesting dynamics, including the spheres-of-influence deal letting Cruz have a clean shot in Indiana and Kasich given first billing in New Mexico and Oregon where a more moderate candidate has a better shot. Yes, that is "collusion", but the main difference between that and a candidate giving up on a state he's badly trailing in is that it was done publicly.
There isn't a rule against it, and if the game is to get to 1237, keeping your main foe from getting there first is part of the calculus. Trump hasn't exactly been all sweetness and light in his campaign and dirty pool is par for the course in his rhetoric; he doesn't have many friends in the process to collude with.
There isn't a rule against naming a running mate in advance, either. It's unorthodox and a bit presumptuous, but conservative saint Ronald Reagan did that in 1976 when he was in Cruz's spot. He named a rather RINOey Richard Schweiker as his running mate as a bid to shore up his left flank and sway unpledged delegates away from Ford. That was before the convention, not in the tail-end of the primary season, so this move is more than a bit novel.
The Carly Fiorina pick isn't of the same cloth as the Gipper's gambit. She was in the race herself, has broad appeal and appeal among broads. That latter pun notes that she would soften any "war against women" attack without having to play against Cruz's Southern Baptist preacher's kid persona.
She might be some help in California, her home base. Not everyone there likes her, as a few folks will have a grudge from HP layoffs, but she does bring a non-trivial fan base that can be helpful, especially when the fight is on a district-by-district basis as well as statewide. Extra operatives can't hurt...at least not too much.
As a general-election running mate, she seems to be a good choice. She's not held elective office, but has been the head of a large corporation, which gives her some executive experience. She's a better speaker than Cruz and gives him a female running mate without it looking like tokenism. Her run for the nomination and a few moments in the spotlight have seen her fare well in the campaign, just not well enough to be first choice for too many folks, but there are a lot of good and talented folks who could say that this year.
Cruz has been playing the small-ball game of getting his supporters named as delegates even if they are pledged to Trump in the first ballot or longer. That's hardball politics and does create a disconnect between the primary result and the delegate makeup, but the rules often have their quirk; winner-take-all gave Trump all the pledged delegates in states where he won only a plurality, so having rules that limit the pledged delegates to being bound for a round or two is another quirk.
It also shows the lack of attention to detail of the Trump campaign. If Cruz is running circles around Trump in getting delegates in place, what would that bode for a President Trump keeping tabs of the details of the US government? You want folks to delegate and not to micromanage too much, but if the details are getting lost in the shuffle, some of the blame needs to flow to the boss.