When In Thy Image was repurposed for TMP, not only did the character arcs for Kirk and Decker have to be developed and resolved within the two hour story, they had to resolve to an outcome that no longer matched the setup they were meant for AND it now had to incorporate Nimoy’s character in a staring role. Decker became a vestigial appendage that while still woven into the fabric of the story, had no function beyond it. These were fundamental story problem that really needed to be solved.
I realized a while back that TMP is basically structured like a 2-part TV episode. Part 1 is about Kirk clashing with Decker, an arc that gets resolved after the wormhole incident when McCoy chews Kirk out and gets him to realize he's been letting his obsession with command blind him to Decker's competence. In Part 2, Decker becomes more of a partner to Kirk while the emphasis shifts to Spock's return and the contact with V'Ger, though Decker still has his own subplot with the Ilia probe.
I believe that only one line of dialogue suggests this: "I remember when you recommended me for this command." But does this have to be construed to mean that Kirk "chose" Decker? Kirk's recommendation would not necessarily have been the sole deciding factor.
Poor choice of words, but my point stands, because it's specifically Kirk's opinion of Decker that I'm talking about. Kirk believed, at the time, that Decker was the best choice to fill his shoes as captain of the Enterprise. It's hard to believe that Kirk would feel that way if Decker hadn't already proven he was fully qualified to do the job.
(Apologies if, among the hundreds of novels I haven't read, there are any that address the post-Kirk, pre-refit voyages of the Enterprise.)
There are none, as far as I know. Despite the one-year discrepancy, it's always been assumed that the Enterprise was out of service for the full 2.5 years that Kirk was at Starfleet Operations, presumably sitting in a hangar awaiting its turn in the refit schedule, or something. In my own Department of Temporal Investigations: Forgotten History, I accounted for the gap by saying that the Enterprise spent much of that year on temporary display at the Smithsonian's orbital annex (a nod to the restored Enterprise miniature at the National Air & Space Museum).