All I really know about this is what I saw in a documentary on the Smithsonian Channel called “Secrets of the Sphinx” or something like that. It rehearsed different theories, including the one about the water erosion. Apparently only a very few amateur archaeologists hold to this position; they believe the sphinx was originally a representation of a lion that later got re-carved into a human face.
As for lining up the pyramids with constellations, I think that is pretty speculative but not out of the question.
