My opinion is that Spock is the bridge between TMP and Khan. His characterization seems to continue where TMP left off. His sacrifice at the end of Khan is kind of the finishing off of the epiphany he had in TMP, about logic not being the be-all-end-all and that friendship (the hand-holding scene in TMP) was more important. There isn't as much of a setup for Spock's transition in Khan alone, merely the Kobayashi Maru theme. But in Spock's very first appearance, he is altogether friendly, offering Kirk a gift, where Kirk is troubled. This may be a trick of the imagination, but when you play both films back-to-back, that is what flows between them.
In other respects, it feels like a do-over, by taking Kirk and putting him back behind a desk, and then reusing the dry dock footage. If Kirk really wanted the Enterprise so much, then certainly he had to have spent some time commanding her again and not been so keen to take another desk-job. So there had to be far more of an interim between the two than the chronological time between the release of the films.