I think the third party is a valid thing to keep in mind. The Republicans are a bit more “ends justify the means”, which translates to not letting themselves get distracted by “perfect is the enemy of the good”. So they might even prefer a third party, but they are less likely to because they tend to be a bit more coldly strategic in their voting.
With respect to they can ignore the results of the primary vote… but that’s exactly the sort of thing that people accussed them of when they put Hilary Clinton up as their candidate. So the right can tear into them for ‘coronating’ their candidate instead of doing an election.
While they can put up someone else, it would be a pretty desperate act, and it’s hard to know which bad option is the worst of the options.
I would suspect never.
Either they are obsessed with their “work” and do it far more than that, or they do nothing.