An interesting post!
I'm just uncertain why you are adamant that the pyramid served a practical function rather than a symbolic one.
If Osiris was venerated as a god of vegetation, his death and resurrection myth serving as a metaphor for the cyclical nature of Earth's seasons, then is it not just as reasonable to assume that the pyramid is a temple and the sarcophagus / Osiris Bed purelly symbolic, also?
Building temples on a grand scale simply to venerate and seek the favours of a life-giving God is an idea as old as the Great Pyramid itself.
Given all the references to myth, deity worship and symbolism to which you refer in order to present your hypothesis, I just wonder how you make the leap to 'recovery vault'.
A people who believed in resurrection surely were not concerned by an Earthly death - were they?