I understand your first point.
2. By “measurable”, I meant “what we can measure”.
Seriously, there must be some general agreement as to how much matter exists/existed in the universe. Don`t computer models of the big bang use such guestimates?
How is the mass of light dealt with in such physics?
Estimates of the universe’s weight vary wildly but there is some general agreement. Slight changes in a big bang model at the early stages will lead to huge changes later in time. I don’t know how the (relativistic) mass of light is dealt with, it represents there knetic energy (E=mc^2) and I honestly can’t say that I know if the combined mass is big enough to take into consideration.