You guys are missing a big hint in that story...
“The force [the Enterprise crew] are met with is much more frightening.
It's Mitchell. He's the character who best fits "frightening force." Maybe Lord Garth, outside odds on Gary Seven, but Khan is less lkely than a herd of tribbles. He represents a threatening idea more than a force, and popcorn movies are poor places to explore ideas. Mitchell and his glowing eyes are tailor made for a summer flick.