Criminy. Earlier I wrote "But if TWoK is construed as a complete reboot starting at time zero, ..." I hate to have gotten into a discussion about (as the former president said) "what the meaning of is is," but when I wrote "is construed" I didn't mean "construed as a reboot at the time"; I know that film producers 30 years ago didn't think in those terms.
I meant "retrospectively considered as a reboot," which makes no more or no less sense than does trying to fit every filmed Star Trek production into a timeline where all events are in fact serialized, even though 99% of the continuity is never revealed to us (and even though there are occasional continuity conflicts that we are shown).
Naturally many fans are inclined to favor the total-serialization way of thinking, hence the two (three?) editions of the Star Trek Chronology, as well as Vonda McIntyre's occasional references to elements of TMP in her TWoK novelization. But if fans can retrospectively place one sort of pattern on filmed Trek, does that make other sorts of retrospective views illegitimate or "conterfactual"?
If the producers and writers of TWoK had wanted to explicitly indicate that X number of years had passed since TMP, that would have been easy to do; they didn't (although they were explicit about the number of years since Khan and Kirk had met before). The initial prints in May 1982 had no "II," as we all know. Kirk starts at a desk job in both stories. There are all kinds of reasons to think that (in modern terms) a reboot was the idea, albeit one with the same cast as the first attempt.
The producers of TWoK, it seems to me, clearly made the correct choice to avoid being an explicit sequel to TMP. I mean, would any of us be here if not for the success of TWoK?
(I'll concede your point about Spock - but I would also argue that Spock's personal growth between TMP and TWoK could have been primarily Nimoy's own doing, and may not represent any effort by the writers/producers/director of TWoK to indicate the lasting effect of the TMP events on Spock.)
Cosmetic differences aside, I just cannot see how TWOK is a reboot (even retrospectivally).
Okay, The uniforms are different and the sets have been given new detailing but everything else is broadly the same. Thematically the movie is a bit different from TMP but there are no massive changes. All the characters are still the same.
It is not a direct sequel in terms of story but it is an implied sequel. For the hardcore fan you can still join the dots between the movies.