Answering my own question
One thing that seems tricky right now — maybe not trickier than it has been for the whole primary contest — is that the main reason to have a Trump alternative is for a scenario in which he physically can't campaign because he's in court so much, he actually gets convicted before the RNC (unlikely, but I guess not impossible) or some other major contingency like that. Then, you'll want a Trump substitute. But Haley, for example, has been running as a Trump alternative. Nothing in the process is really conducive to such a fine distinction, even if it were clear that a critical mass of Republicans were invested in either a substitute or an alternative.
—Julia Azari, 538 contributor