Having a white Khan with a British accent is like having a black Sulu with a Bronx dialect. That would be stretching but it would work, wouldn't it?
As opposed to a Latino Khan with a somewhat Spanish accent?
I guess it would be as believable as Jean Luc Picard's accent.
The difference is, the modern PC factor. In the 60s, that wasn't an issue and anyway, who the phlox could tell the difference between a Mexican and an Indian? You could not not not get away with that nowadays, not for a Scalosian minute.
Cumberbatch could be a brit who identifies himself enough with the Indian culture that he even takes an Indian pseudonym.
That's too much grounded in modern assumptions and national identities. Trek don't do that stuff. Also, it would require too much explanation, starting with the fact that India used to be a British colony. You
really don't want to assume the audience already knows that, and in a two hour summer popcorn flick, it's too much clunky dialogue when the audience is getting antsy for the characters to shut up and shoot someone already.
But Cumberbatch playing some random resident of Earth with white skin, who is a follower of the long-ago infamous augment, Khan Noonian Singh, and taking the name Khan like it's equivalent to Caeser, could avoid all the problems. In that case, the lack of explanation works in their favor. How many people in the audience would know that Khan is an Indian name at all? They might assume it's just a made-up name like Spock. As long as the story isn't claiming the guy is Indian, the PC factor won't kick in. They don't really need to say where on Earth the guy is from at all.
The less explanation and backstory, the better. Whittle it down to some simple statement - "Romulan miner, pissed because Spock let his planet blow up, wants revenge" or "Follower of crazy superpowered guy from future Earth history thinks he's better than everyone else, wants to take over" - that's all they really have the bandwidth for.
