I think it works as a one-off because that makes it a coincidence. And it helps with the audience, especially back then, to have someone who knew Khan and could fill people in. Even though the person they picked wasn't there, which moots the point.
Made sense in context of the movies. Assuming the Vulcan only has one Federation Ambassador, or a go-to, Sarek works.
Once in a fantasy in "Shore Leave" is fine, and once as a story point works, after that, it's stretching it. He had, I think, three live ex's show up in the series and one in the movies. But if we stuck to realism, we'd never have had Bibi Besch and that would have been a genuine shame.
Made sense in context of the episode - Spock went back home to see her.
Never saw her. Never mentioned either. In fact, if you go by only the aired live action episodes, you'd never know McCoy was ever married or had a kid. The first mention of a daughter at all was in TAS "The Survivor." Unless you mean the tie-in media but I'm assuming you're not because: