This is indeed uncharted territory.
But with the Free the Delegates movement continuing and even gaining steam, it is not impossible that he could be defeated at the Republican convention, or that he might choose to withdraw at any time either before or after the convention.
As for "resigning" after election, one would think Trump would have issued an ironclad denial. This he did not do. Read more here: < Trump Could Resign Before Term Begins, If Elected >.