Barbara Marciniak mentions Reptilians being involved in Mesopotamia in ancient times. Was that correct, and were they there as members of an alliance with the Anunnaki?

Both are true. The Reptilians have been present within the sphere of human affairs since the time of Atlantis. So, they have been coming and going, and participating in the manipulation and governance of humanity and its exploitation, on an ongoing basis, for tens of thousands of years now. So, the regimes, for all practical purposes, are over-lapping because the extraterrestrial presence of all races of beings currently subjugating humans have been part of an alliance for a very, very long time.