I'm really leery about your theory of ammonia plants. I don't negate that they choose sites for their sources of natural gas. But, what about the fertile silt laid by the Nile? Egyptian were obviously smart, so why didn't they use this fertilizer on site or around? For what I know there is no trace of fields around.

About the sheer size of the pyramids and the different shapes of pyramid through ages, they seem to follow the Mesopotamian mastaba. Still now powerful kings and presidents usually like great buildings at whatever expenses.

My last point is about stone cutting and transportation. There is a theory that Egyptians didn't need to carve and carry these blocks because for the most of it they cast blocks of concrete on site. When we look at the Roman Pantheon built 2k years ago and perfectly upright , we can cry with our 50 years concrete lifespan. I'm sure they got their recipe from the Egyptians :)