Is God the LionTurtle?
Nope! This kind of question is why I want it to be a group writing effort. The current Avatar was raised Christian, so zie has a certain background knowledge on how that religion interprets bending/the Avatar. A Muslim Avatar, Jewish, Avatar, or an atheist Avatar would see things completely differently.
As for the lion turtle, there are no hybrid animals in this AU. The closest I could think is them being in the Spirit World or being featured in every world mythology like dragons are.