I'll just roll with it like the way Klingons went from their TOS appearance to their TMP appearance with no explanations. They had always looked that way. In my mind I'll just pretend Kahn always looked like Cumberbatch. Of course, the Klingon appearance got a retcon and maybe nuKahn will too someday. Until then I won't lose sleep over it.
Yeah for me this is a non-issue. The exact looks or ethnic backgrounds don't need to match. If the actor is good, he'll make it his
The genetic engineers spliced some Time Lord DNA into the Augments. Cumberbatch is the Second Khan.