The LEGO Group explored the South American cultures in an Adventurers sub-theme. A pity for Pirate fans though, because it could have made an interesting Pirate sub-theme (like you suggested) with Imperial Armada mini-figures instead of Johnny Thunder and Co.
I don't think we have enough posts for a debate yet. But I think the debate Norrington is imposing is the Islanders aren't aware of the Imperials Guards existing. But think I it's a moot arguement because this is Enoa's MOC so he determine the back story and thus decides if the Islanders are aware of on the Imperial Guards existence.