Why not get a rental car? They're so-o-o-o cheap in Orlando and they give you a lot of flexibility. I think by the time you add up Mears or Tiffany transportation from the airport and back and from Universal to Disney, you'll find a rental car is not that much more expensive.
Reservations are not required at Mythos, but now that attendance is picking up again, I'd advise it just in case. It would be a shame to miss Mythos.