Re: Time Frame for Appearance of Superhumans
I like them all, really. I really enjoy a well-put together timeline. I also like the idea of the PCs being among the first superheroes to debut. As a GM, it depends on the kind of world I think suits the players and my interests best at the time.
Just to mention another setting, the Omlevex setting had supers appearing at least before the American Revolution, and operating openly during the American Civil War. The idea was that such conflicts pretty well diminished the supers populations, keeping the numbers down over the years.