And it turns out that the utilitarian approach of adding up utilities is *not* a bargaining solution, because it violates Pareto-optimality in some cases. Does that “disprove” total utilitarianism?
I’m not sure this is right. As soon as you maximize a weighted sum with non-negative coefficients your solution will be weakly Pareto optimal. As soon as all coefficients are strictly positive, it will be strongly Pareto optimal. The axioms mentioned above don’t imply non-negative coefficients, so theoretically they are also satisfied by “anti-utilitarianism” which counts everyone’s utility negatively. But one can add stronger Pareto axioms to force all coefficients to be strictly positive.
The problem with the utilitarian Bargaining solution is that it is not independent of affine transformations of utility functions. Just summing up utility functions is underspecified, one also needs to choose a scaling for the utility functions. A second criterion that might not be satisfied by the utilitarian solution (depending on the scaling chosen) is individual rationality, which means that everyone will be better off given the bargaining solution than some disagreement outcome.
Your argument seems to combine SSA style anthropic reasoning with CDT. I believe this is a questionable combination as it gives different answers from an ex-ante rational policy or from updateless decision theory (see e.g. https://www.umsu.de/papers/driver-2011.pdf). The combination is probably also dutch-bookable.
Consider the different hingeynesses of times as the different possible worlds and your different real or simulated versions as your possible locations in that world. Say both worlds are equally likely a priori and there is one real version of you in both worlds, but the hingiest one also has 1000 subjectively indistinguishable simulations (which don’t have an impact). Then SSA tells you that you are much less likely a real person in the hingiest time than you are to be a real person in the 20th hingiest time. Using these probabilities to calculate your CDT-EV, you conclude that the effects of your actions on the 20th most hingiest time dominate.
Alternatively, you could combine CDT with SIA. Under SIA, being a real person in either time is equally likely. Or you could combine the SSA probabilities with EDT. EDT would recommend acting as if you were controlling all simulations and the real person at once, no matter whether you are in the simulation or not. In either case, you would conclude that you should do what is best for the hingiest time (given that they are equally likely a priori).
Unlike the SSA+CDT approach, either of these latter approaches would (in this case) yield the actions recommended by someone coordinating everyone’s actions ex ante.