I was eventually seduced into reading the whole thing after some skimming, but it was a close thing for a while. I think this piece could be shorter (footnotes if you can't lose anything), even though the writing is solid already.

I like your approach.

Vulnerable points:

1) What it means to "average" or "sum" the utility functions of individuals needs definition. The way in which individual utilities are made comparable isn't obvious to me. I feel like we either have to declare some popular components of utility as the basis for normalization (define some common ground), or provide a framework where individuals can consciously elect any utility function (we believe what they say, because we have no perfect lie-detector) while providing a combination-of-utilities that can't be gamed by lying (this is probably impossible).

2) "average utilitarianism is perhaps interpretable as using the metric in which the new people are not realized" - as you know, it's only like that when the new people will have the same happiness on average. But it's certainly more like what you say, when compared to total, than not.

3) Reasoning about recently killed people based on your instructions does seem to require care or at least hand-waving :)

4) I think you're saying that if we currently expect to have a certain demographic of extant humans N years in the future, then we should weigh what we expect their utility to be in our decisions now (with some discount, considering them equally with living people). I guess you'd say that this should change my decisions (or at least my vote in our joint utility-maximization) even if I don't expect to personally reproduce. But if we decide to embark on a course that will change that demographic (e.g. measure that decrease birth rate as a side effect), then we no longer need to consider any utility for the now-not-expected-to-exist population. This actually makes sense to me, in a "you break it, you buy it" sort of way.

4a) Assuming I understand you right on 4, I feel (with no underlying formal justification) that if e.g. the Amish decide to reproduce such that we expect them to be half the population in 100 years, then the expected personal utility of that half of the population should be weighed at less than half of the 100 years from now population (i.e. less valuable per capita). This may just be my selfish genes (or anti-Amish bias!) speaking.

5) How is what you advocate not just average utilitarianism?