I think the stupidity of utilitarianism is the belief that the morality is about the state, rather than about dynamic process and state transition.
“State” doesn’t have to mean “frozen state” or something similar, it could mean “state of the world/universe”. E.g. “a state of the universe” in which many people are being tortured includes the torture process in it’s description. I think this is how it’s normally used.
Because people are running on similar neural architectures? So all people would likely experience similar pleasure from e.g. some types of food (though not necessarily identical). The more we understand about how different types of pleasure are implemented by the brain, the more precisely we’d be able to tell whether two people are experiencing similar levels/types of pleasure. When we get to brain simulations these might get arbitrarily precise.