It's an interesting theory. I did find it kind of weird that Moriarty would reveal himself to Sherlock just to give him a warning. So maybe Jim is a front man... They could go either way, really. It'd be a good way to bring back the character if they ever kill off Jim.

And I totally didn't realize Moriarty only appeared in one Sherlock Holmes story but he was a criminal mastermind, too, so the show really didn't change the character. See, I need to read the rest of the stories.