Excellent paragraph IMO. However, what makes spacetime spacetime? I would regard the spacetime variables as meaningless in absolute terms and that we simply define the given variables as spacetime variables. Thus, when we measure energy-momentum, we are constructing the instrument in terms of the given variables, and the energy-momentum we measure is relative to the given variables. In other words, energy-momentum has a definite relationship to spacetime even if energy-momentum and spacetime are meaningless in absolute terms. That is, quantum mechanical operators are relative to the spacetime operators.