I don't think Cumberbatch is playing Gary Mitchell. He died in the comics. BC's mother reportedly said that he was dressed in leather, head to toe, for much of the movie. I'm hard pressed to think of a TOS character that was dressed all in leather (not hides). BC could be playing an android, but I'm not going with that one either.
I don't think he's playing Khan either, but that isn't saying Khan will not be in the movie. Del Toro was approached for a role that he turned down, but I'm thinking the reason he turned it down (his mistake) was because the dollar amount was not suitable, indicating it was a very short term role. The Khanists will get their Khan so they can stop haranguing the production staff and the rest of us, about it.
As for Eve's role, I don't think it's Dehner. I would love to see Chapel instead of hearing her, this time, but what I don't want to see is her blindly chasing after Spock, only to be rebuffed. Rand is back on the table, how about that.
What I want to see is more strong female characters.
Rand is back on the table?!?! Where did they say that? That's awesome. The two canon characters I, at least, want to see the most are Rand and Chapel.
And, hmm, the whole leather-clad thing could
mean Khan, could it not?