After having read all of your theories, allow me to tell you mine. The person who wrote the letter is obviously a person who knows the legends very well, yet is not one of them. My idea is that "Smartypants" is either "Syndicate Boy" (from Bangalore's S5 loading screens, perhaps he's the one who admitted them into the Games - which explains why the writer knows them well), either it is "Tech Boy" (Jamie - the blue haired guy, after all he was the one found Revenant, and judging by him being a hacker, he must have found out things about the legends). If this seems far fetched or not plausible, then I don't have any other ideas 😅