It is also worth noting that average utilitarianism has also its share of problems: killing off anyone with below-maximum utility is an improvement.

No it isn't. This can be demonstrated fairly simply. Imagine a population consisting of 100 people. 99 of those people have great lives, 1 of those people has a mediocre one.

At the time you are considering doing the killing the person with the mediocre life, he has accumulated 25 utility. If you let him live he will accumulate 5 more utility. The 99 people with great lives will accumulate 100 utility over the course of their lifetimes.

If you kill the guy now average utility will be 99.25. If you let him live and accumulate 5 more utility average utility will be 99.3. A small, but definite improvement.

I think the mistake you're making is that after you kill the person you divide by 99 instead of 100. But that's absurd, why would someone stop counting as part of the average just because they're dead? Once someone is added to the population they count as part of it forever.

It is also worth noting that average utilitarianism has also its share of problems: killing off anyone with below-maximum utility is an improvement.

It's true that some sort of normalization assumption is needed to compare VNM utility between agents. But that doesn't defeat utilitarianism, it just shows that you need to include a meta-moral obligation to make such an assumption (and to make sure that assumption is consistent with common human moral intuitions about how such assumptions should be made).

As it happens, I do interpersonal utility comparisons all the time in my day-to-day life using the mental capacity commonly referred to as "empathy." The normalizing assumption I seem to be making is to assume that others people's minds are similar to mine, and match their utility to mine on a one to one basis, doing tweaks as necessary if I observe that they value different things than I do.