Maybe he's a Japanese Canadian. His english would be fluent, it would explain his use of British spelling, and even the double space after a period.
I think Satoshi is an individual person. He writes with a consistent style. I doubt he's American, since he sometimes uses British spelling.
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