One of the recurring questions of course was how did Chekov known Khan if he didn’t show up in the series after Space Seed? I think again the Trek Novels had this down in “To Reign in Hell” Book 3 of the Khan trilogy we see that Chekov had been serving on the Enterprise, in security. He was part of the party that gave Khan his supplies to survive Ceti Alpha V and build a community (including 1 hand phaser as I recall) and he was sure to remember who were among the ones that exiled him there.

Now as to why he didn’t tell Terrell about Khan until it was too late, it turns out that the whole incident was classified by Starfleet. No need to have people know that the ones mainly responsible for the Eugenics Wars of the 1990s might have a colony out there. Dr. Marcus search for a viable candidate planet included dozens of systems and only should this one look like a viable candidate would Chekov then be forced to tell Terrell as to why not. There was no reason to expect that Ceti Alpha VI would be the right choice with Dr. Marcus’ extacting demands.