I'd like to comment on something from your post:
> The body of the Sphinx has been buried several centuries
> longer than the head, yet the head is in better condition
> than the body. Some geologists say this is due to the upper
> layer of sandstone being of a better quality than the lower
> layers that make up the Sphinx' body. While it may be true
> that the head is of a more weather resistant stone than the
> body it doesn't fully explain how the head could have
> survived with most of its features intact while the
> stratified body, as one ancient grafitti writer put it,
> resembles a loop of yarn.
It's not correct that the Sphinx does not suffer from erosion during the time that the monument was covered in sand. A different kind of erosion could have taken place, as presented by James A. Harell, professor and chairman of the Department of Geology, University of Toledo.
On this page you can read:
"In 1994 (4) I invoked what has become known as the "wet sand" hypothesis to explain how subsurface weathering might have affected the Sphinx since the time of Khafre. The sand filling the enclosure and lying in contact with the walls and Sphinx body must have been wet much of the time. This was certainly the situation when the sand was cleared from around the Sphinx in the early 1980's. At that time it was observed that the sand was "completely soaked with water a few inches below the surface [and also that] the bedrock in contact with the sand was soaked" (11; see p. 4-5)."
Then he continues to explain the rainfall in the region and concludes:
"Thus, it seems that every several years, if not more frequently, there were rainfalls that wetted the sand in the Sphinx enclosure. Depending on the intensity of the rainfall, the water would have arrived either mainly as overland (surface) runoff or subsurface flow. Most of the water will enter the Sphinx enclosure around the western walls and so, understandably, it is here that most of the weathering and erosion has occurred. Once soaked, the sand would remain wet for many weeks or months due to capillary retention."
For a more detailed view check the link I provided with this and read the part "Subsurface weatering"