Here's the thing: Even if they put out a press release tomorrow announcing, once and for, that Cumberbatch was playing Gary Mitchell (or whomever), what practical difference would it make?
Would millions of previously undecided moviegoers suddenly circle the opening date on their calenders? ("Well, I wasn't planning to see the movie, but if it's Gary . . . well, sign me up!")
Would millions of devout Trekkies suddenly decide to skip the movie after all? ("I loved the last movie, but Gary Mitchell? That's a deal-breaker!")
Would hordes of disappointed Garth of Izar fans storm the Paramount lot, forcing them to reshoot the movie so that Gary becomes Garth? Would they have to digitally turn Alice Eve green so that she looks more like Yvonne Craig than Sally Kellerman?
Would knowing the villain six months in advance have any impact whatsoever on the final reviews and box office? Of course not.
All that would happen would be a flurry of news articles, followed by six months of heated debate about whether Cumberbatch is right for the part--and whether Garth or Khan would have been better. Which could be entertaining, but inconsequential.
In the end, the movie will succeed or fail on its own merits. And we won't know that until May . . ..