Spock was the only character to have his own model kit during the first run
I agree with everything you're saying except this one technicality: the Spock kit came out in 1974.
The 1974 kit was one of AMT's many reissues. The first Spock vs. 3-headed serpent kit was released in 1968 by Aurora Plastics in the UK, but licensed by AMT for North American release, while AMT was responsible for all other TOS kits (starships, Galileo shuttlecraft, Command Bridge, Exploration Set, etc.).
With Spock being the only screen accurate figure (sort of)--of any kind--made while the series was in production, Nimoy must have felt an astounding amount of power, but i'm sure Doohan, Takei, Nichols and Koenig were looking the other way, or on some extended coffee break when big Nimoy developments happened...
...it was all about that darn devil Shatner--the Grand Manipulator of All Things Star Trek.
I built one at the time. I painted the boulders metalic gold and silver, as if the three-headed serpent was guarding a treasure.
If you have not spotted the old Keith Meyer Space Station
kit site (ton of modeller galleries), take a look at the fantastic Spock kit build-up from a fine modeller named S.M. Clark:
Back in the day, I could only wish my Spock kit turned out like that!