I think Satoshi is an individual person. He writes with a consistent style. I doubt he's American, since he sometimes uses British spelling.
if im understanding that right.. he could literally prove who he is by using his private key to a sign a message and send it via the client(?) to everyone!
Other people have that alert key now. However, Satoshi has published a PGP public key of his own:
https://forum.bitcoin.org/Satoshi_Nakamoto.asc