Regarding Reliant not knowing the difference between Ceti Alpha V and Ceti Alpha VI... it just occured to me that Chekov doesn't think to mention Khan until they get down onto the planet. Sure, it's been 15+ years, but surely if one of humanity's most badass super enhanced villains (who actually brought Earth to its knees) was once stranded on a planet in the Ceti Alpha system, you'd think Chekov would at least tell Captain Terrell and the rest about him. Instead it's like he sees the Botany Bay name and he shits his pants. "Oh fuck, I knew
there was something I'd forgotten about this star system!"
Basically the entire plot hinges on Chekov being a bit of an idiot.
I rather like that the TWOK novelization mentions something of this nature, and also states that CA6 was a moon
of CA5, rather than a distinct planet in its own right. While that makes the naming convention more confusing and silly, I think it meshes better with Khan's description of how the explosion wrecked the environment on CA5. Had another full planet exploded, I don't think they would have survived at all. CA6 was described as being geologically unstable, a fact unknown to Kirk's crew because the area was poorly charted and largely unknown in "Space Seed." Chekov was part of the crew that helped relocate Khan's people to the planet surface.
Chekov does note in the novel that the Reliant is getting inconsistent readings from the system, which is still barely explored as late as TWOK, and he doesn't fully connect the dots until Khan captures him and Terrell. Course, the fact that dialogue suggests Chekov and Terrell mistook CA5 for CA6 kind of goes against this theory.