Just because WE didn't see Chekov doesn't mean he wasn't on the ship.
Well thanks to JJ Abrams, nothing is true anymore. :lol
Well OK I grant you that there's no specific dialogue in a Star Trek episode that establishes when Chekov joined the crew. Perhaps in the first season, Chekov was always just out of frame, in the next corridor, or had just left the room. Probably Khan ran into him in the men's room. :rolleyes
I will say though, that Walter Koenig has said that he DID know that he had not been working on Star Trek during the first season, so therefore he was not in "Space Seed" and the script for WOK was in error. He claims to have very wisely NOT mentioned this to anybody, because he figured they would just shift over all his lines to Sulu and he would lose out on a plum role. :lol
As to the "technological barbarian" look of Khan and his people, yeah that is a nice touch. After Ceti Alpha Six exploded, "laying waste" to CA5, Khan had nothing but the contents of the "cargo bays" to sustain him.
Incidentally I can't think of any way that Ceti Alpha Six could explode such that Ceti Alpha FIVE could be misaken for that planet. Just another "don't stop to think about it" moment in Wrath of Khan. :lol
- k