It was the same actor so I assume it's young Jacob! Not everyone, trésor, Richard didn't see the kid, did he?

Yes our Not!Locke still thinks of himself as the MiB but I think he's sorta possessed/influenced by Locke's mind too. He obviously has all John's memories, and we could wonder where the difference is between remembering being someone and actually being that person.

BTW it could explain his "protectiveness" towards Claire. It isn't like she's useful now.