Crazy idea. What if the alternate Carol Marcus has a different mother?
If Admiral Marcus wasn't involved in Section 31 in the original timeline, or simply if Section 31 didn't have a large presence in London, since the Kelvin Memorial Archive mightn't exist in an unaltered timeline anyway, perhaps he met and married a different, British woman due to visiting London more often. It's never explicitly said when Carol was born, and indeed if anything she seems younger than Kirk.
So, the alternate Carol might be really