Well obviously the plot was adjusted to fit the new character.
How is that obvious? Is there any evidence to support your claim that Noel was specifically intended to take Rand's place in the story? You're stating this as a fact, but what are your sources?
The dynamic with Noel was clearly that familiar morning after dynamic that arises from far too many office parties. It was speaking to the audience with its tongue in cheek rather than saying a lot about Kirk's morality.
I don't think it's "clearly" anything of the kind. And I'm not talking about Kirk's morality, I'm talking about the consistency of his characterization in the first season. I'm talking about the James Kirk who was the only human male on the
Enterprise who was uninterested in Mudd's women because he was so fixated on his duty, who saw them only as an annoying disruption to the ship's operation. The James Kirk who objected to being assigned a female yeoman and who consciously avoided showing any romantic interest in her even though she wanted it. Who, when intoxicated by the Psi 2000 virus, lamented that his duties wouldn't allow him to get involved with a woman. Who only seduces Lenore Karidian as a manipulative ploy to expose a murderer. Who only needs ten seconds to shake off his illusory love for Helen Noel and coldly order her to risk her life. This portrayal is consistent throughout the season, and in the context of that, the logical interpretation of "Dagger of the Mind" is that Kirk was so driven, serious, and professional toward his crew that he was embarrassed merely for having danced and flirted lightly with a woman under his command.
Today we see Kirk in a different way because of how his characterization was adjusted later on by network pressure (and perhaps actor pressure) to make him more of a romantic lead. But at the time "Dagger of the Mind" was written, that is not how James Kirk was perceived by the writers. At the time, only ten episodes into Shatner's tenure in the role, they were basically writing him the same as Christopher Pike -- duty-driven, serious, full of self-doubt, keeping his more personal urges buried under his military discipline. It was the influence of Shatner's personality over time that evolved him into a more distinct character, but this early in the process, that evolution had barely begun.
Besides, Helen Noel's dialogue is unambiguous: "At the Christmas party, we met, we danced, you talked about the stars. I suggest now that it happened in a different way. You swept me off my feet and carried me to your cabin." It doesn't get any clearer than that: nothing happened at the Christmas party. Hell, I doubt the network censors would've allowed the episode to get away with claiming otherwise.
I still think it's a shame that post-TMP didn't put any effort into allowing the charcters' relationships outside the crew during that intervening 15 years.
Well, Sulu did manage to have a daughter somewhere in there. And we've established that the TMP-TWOK gap is quasi-officially set at 12 years, not 15.