as for who the real mysterion is, someone posted this on yahoo answers:
There is a big clue that everyone seems to have overlooked- the fireworks used in Mysterion's escape from the Coon, as Cartman pointed out "they are illegal"- who has a history of illegal firework smuggling in South Park..? That's right- Stan's uncle Jimbo ("Summer Sucks"-S02E08), that's how Mysterion could get hold of illegal fireworks.
On top of this, for total proof- And as previously mentioned the "M" stood for Marsh. Kenny couldn't afford fireworks. He visited Kyle (counts out Kyle- but they've got to be good friends). Clyde was asleep in class when Cartman revealed the Coon's location. And Cartman didn't actually *say* he thought it was Wendy- when she suggests it need not be a boy he just exclaims "f*kin b**ch".. Also, Stan is unpredictable and highly secretive at times, like in "The Urinal Deuce"."
and something i noticed is professor chaos said that the body type had to be someone like stan or kyle, but the keyword here is "like" so that evidence isn't the best