OK, so Drake and Josh existing in the Schneiderverse is fine, because the show is never seen being played within the Schneiderverse (you can only hear it), so it could be that Schneiderverse version of Megan is played by a girl who looks nothing like Carly. BUT how is Tori looking exactly like Shelby Marx explained?

Since alternate universes have been shown to exist within the Schneiderverse, the iCarly episode 'iFight Shelby Marx' took place in another universe. That's my explanation for there being two 'Toris'.