This is a lot of speculation on what candidate or scenario would make the Republican more electable in November. Every one could be argued without proving anything.
Trump being handed the nomination now could be the worst possible scenario. He loses out on valuable campaigning experience he could gain in the remaining states. Trump is learning from his losses how to be a better candidate. We’ve seen him make serious shifts after his Wisconsin gaffes and loss.
A contested convention might not be a “nightmare scenario.” The deals that could get cut at a contested convention could be structured in a way to make all sides happy and unite the party.
The anti-Trumpers are a factor. Can you win the general election without getting them on your side? If Trump improves himself as a candidate by campaigning through California and then throws them a bone at the convention to get their support to win the nomination, then he could emerge much stronger than if he were to be given the nomination today.
Besides, if Cruz dropped out now, Kasich would stay in, and would be in a position to consolidate the anti-Trump vote and gain delegates, possibly putting the establishment in a stronger position at the convention if Trump still doesn’t make 1237.
Cruz need not end his campaign.
However, I would say after Indiana, he could make the accommodation with Trump, end all of the nastiness, and proceed forward knowing that at the convention, if Trump needs the support to get him over the top, Cruz could provide it.
...and starting then, (after Indiana) reset the tone and begin talking about a united GOP for November, whomever the candidate is, including Trump.
What is needed is for Sruz and Trump to come together, end the war, and begin making comments in the press indicating that they are coming together.
Cruz can do all of that without officially ending the campaign...but still facilitating a united party behind Trump when he goes over the top.