Thanks for the helpful feedback!

1) Yes how we measure utility is always an issue. Most papers I've read don't address it, working off the arguably fair assumption that somehow there is greater and less utility, and that anything in real life is just an approximation but you can still shoot for the former. Ideally we would just ask trustworthy people how happy or unhappy they are, or something similar. In practice and for prescribing behavior though I think we use the popular components approach, assuming most people like food and hate being tortured.

2) I'm slightly confused by this. Are you talking about bringing a large group of people into existence, with varying utilities? For simplicity I was discussing ideal theoretical cases, such as one child, or yes a new population all of roughly the same utility.

4) Yes that's essentially my point, though I haven't (I think) yet suggested how realization of these "decision-changed metrics" alters our decisions about potential people. But perhaps you meant simply that the wellbeing of someone who will exist should affect what we do.

4a) I would say that we should treat all people's utility equally once they come into being, which I think agrees with what you said. The last line about anti-Amish bias seems to run counter to that idea however.

5) Before a normative rule came to me, I was going to end this post with "lacking any prescriptive power however, we might default to total or average utilitarianism". Regardless, I've tried to keep this post merely descriptive. Though the rule I came up with is similar to average utilitarianism in ways, average utilitarianism has consequences as well I'm not happy with. For example, if there were 20 people with extremely high utility and 1000 with utility half that but still very good lives, as long as those 20 people didn't mind the slaughter of the thousand, the average approach seems to advocate killing the 1000 to bring up the average.