There’s also nothing that indicates Kirk didn’t serve on the Enterprise in another role before getting promoted
Hm, here's an interesting formulation for all or part of a final season:
- Pike leaves, becomes an Academy instructor
- Una becomes captain of the Enterprise
- Kirk transfers in to be her XO
Kirk and Spock working together for a year or so would give us a chance to explain the unusual situation where Spock is simultaneously science officer and XO. By the end of this season you'd have the full TOS crew in place. (Minus perhaps Chekov, or maybe he's a cadet like Uhura was in season 1.)
Other than the Kelvin timeline where they said up front "THIS IS A DIFFERENT TIMELINE," when has that ever happened?
Keep in mind, we're talking about showrunners who contrived a reason for Pike to be "fleet captain" for a single episode just so they could have Kirk and Pike interact without invalidating one line from TOS: "Court Martial." These showrunners really strike you as fans who are going to make "minor adjustments" and justify it with "well you see back in S02E03 we changed the timeline, so now we can do whatever we want!"