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.
That's not how it works, I think. Planets are dozens or hundreds of millions of kilometers from each other. The way Khan describes it, the explosion shifted the orbig of Ceti Alpha V, presumably from gravimetric changes due to the planet's destruction.