Ah, we had met Chekov before this episode, several hundred stardates before in fact: his introductory "Catspaw" apparently takes place before "Space Seed" in-universe.
From there, it's not a big leap to accept that
everybody on the ship got Khan's autograph, many insisting on anatomically challenging locations and tattooing gear. The budget of the episode just didn't allow them to show more than a handful of the groupies, but the message was clear enough: only Spock could fight the irresistible charm of the superman.
If Khan and his band of multiethnic adult followers were stranded 15 years ago, how come all his followers are now white and in their 20s?
Because they are genetically perfect fifteen-year-olds, all born from the dominant marriage(s) on the colony?
Let's remember that for all the talk, Khan's followers in "Space Seed" all represented the generic western ethnicity (albeit with dark hair more prevalent than blonde). If Scotty was right and there were other types, they were apparently all slaves to the top clique and toiling in menial tasks off camera...
For that matter, how come Khan himself was no longer made up to look Indian?
He got sandblasted before throwing together that cool mask.
How could Chekov and the Reliant crew have no knowledge of Khan being stranded in the Ceti Alpha system
Chekov demonstrably had the knowledge, and Khan deduced on screen that Chekov had kept the facts from Terrell - and Kirk thus supposedly from Starfleet. Not much mystery there.
how could their observational skills be so inept that they couldn't tell the fifth planet from the sixth
How could anybody? If there are five planets, then the fifth is the fifth, not the sixth, with one world arbitrarily left uncounted! One could just as well ask why Earth isn't correctly identified as the fourth rock from Sol by Tom, Dick, Harry and Sally, what with Vulcan (the real one) once having been the first even if later developments made that unhappen.
Or are we assuming that the heroes could uniquely identify a planet whose primary attribute in the movie was that all its attributes had changed? If Ceti Alpha V had changed its orbit and become a desert after Ceti Alpha VI had exploded, odds are that there wouldn't be anything recognizable about the star system's planets, and no good way to tell whether the desert world had been Ceti Alpha I, II, III, IV, V or VI previously.
Had the system been charted in detail, Kirk wouldn't have dared strand Khan there. And had Starfleet possessed the ability to monitor distant star systems in real time for cataclysmic explosions and the like, we wouldn't have had "Doomsday Machine".
I mean, take Marla McIvers for example. She didn't just magically appear on the ship in time for "Space Seed."
...Kirk even referred to her in terms that suggested that she was a longterm nuisance. Chekov in contrast was spoken of as a newcomer in his first onscreen appearance, so it's a slightly different case from either McGivers or some random Joe Tormolen who was not given a background one way or the other.
Timo Saloniemi