Wouldn't the same weight bullet/same velocity pressure curves of something like the 416 Rem vs 416 Rigby show the 416 with a higher shorter duration curve for the Rem vs a lower longer duration pressure curve for the higher volume case of the Rigby??

Is "tonnage" or cup/psi etc just suggestive of the peak pressure of each round?

I think that is correct.

However, a consideration is that it will require a certain amount of pressure for something "to give". For example, one hot load and you have a loose primer pocket but 20 shots of mild loads and the primer pocket remains tight.

Average pressure or pressure under the curve would increase with a longer barrel