It's hard to tell, like I said, from these pics, but it really looks like the scuff mark is absent from his right cheek in the pics with Uhura and is there in the ones with Spock.
I'm definitely not saying it is for sure Khan. There could be a thousand reasons why he could break free of the neck pinch, but him being Khan can't be excluded from those reasons. I'll also grant you, Spock did successfully neck pinch one of Khan's followers in Space Seed, if memory serves. I could be mistaken or am possibly confusing it with another episode though. Haven't watched Space Seed in some time. Definitely going to be a blast trying to put all the pieces together though!
But if it is Khan, it's a possibility that he and his followers don't have any clothes on at all if and when they are found in cryogenic freeze. I can't imagine being frozen with clothes on. (Yes they had clothes on in Space Seed, but that was network TV). Seems at least plausible. That would explain the borrowed uniform.
Based only on the tidbit we've been shown so far, it would be a logical fallacy to be 99.9% sure of anything other than Spock and Uhura are tangling with a baddie of some sort who is able to overpower a Vulcan administering the neck pinch even after, presumably, having taken a phaser blast at some point prior.