2. That the ancient inhabitants of Atlantis would have made sacrifices to the underground Chthonic races fits perfectly with our map, as they are directly under Atlantis, Mu and Lemuria on our map. They would control two resources, Vrill (which will eventually attract the U-ga to earth) and the Necronomicon, which enables powerful dark spells.
In the end, we only have to rename the underworld cities to associate them with the Chthulhi (Kalkar and Luthan are two obvious choices), and find an appropriate leaderhead for them (Chthulu?). As for the U-ga, we can add some Angels, to bring them in line with this and with ERB's description of them.