I'm befuddled at the average utilitarianism thing.

First -- how could the truth of a fact of physics (big worlds) ever be relevant to the truth of an ethical theory like average vs total utilitarianism?

Second -- "You just want the average quality of life to be as high as possible, in the future worlds that are your responsibility." is *not* AFAIK the same thing as average utilitarianism; average utilitarianism would average across everything, including what's not your responsibility. This matters for concrete prescriptions.

Third -- suppose someone has an extremely high quality of life, but a bit lower than the average; are you really going to tell him you regret his being born? It just seems absurd.

Fourth -- it sounds like average utilitarianism requires an unambiguous binary way to decide whether you're a continuation of some past person-stage.