How come Khan's followers, who were multiethnic adults
Where they? I haven't seen the episode in some time but screenshots seem to show a bunch of white dudes but like I said I haven't seen it in ages...
Regarding the Ceti Alpha V / VI debate, my view is that Starfleet saw a big bang and assumed that Ceti Alpha V had blown up which is why no-one checked on Khan in X number of years and why Chekov had no problem hanging around in that system.
CHEKOV: You lie! On Ceti Alpha Five there was life, a fair chance.
There WAS life... past tense. He thought it had blown up.