Cumby is the human side of Spock!
Cumby looks just like a human version of Spock, the body and the dark hair. So what if the 'Enemy Within' happened with Spock and not Kirk?? Spock was split into a Vulcan side and a human side (Cumby). It's and interesting senario. It also explains why JJ can't publicly name the villian yet.
Wouldn't Spock's human half look like Zach Quinto? (or at least Winona Ryder?)