Hi B A Hokum
I must research to answer the two excellent geological questions you ask.
Since there is presently no evidence of long term storage of blocks once cut I would assume the AE waited till imminent need to cut stones.
I do not believe we can be sure who the face is to represent, as it is as likely Khafre was honoring Khufu as himself, for all we know it could be his missing (from that location) brother Djedefre getting his spotlight in the family crypts.
I believe it was executed around the time of Khafre, but allow the basic Sphinx area could have pre-existed. Not by 7,500 years though.
What King/project would you attribute the original Sphinx work too?
Jammer