Unknown:

Besides (in the usual single world): is Eliezer willing to kill off everyone except the happiest person, therefore raising the average?

No. Because that creates Death events, which are very large negative utilities. It increases the average number of people who experience short lives.

I have a suspicion that when all is said and done and known, quantum immortality is not going to work out.

It looks to me like Eliezer already preferred average utilitarianism even before knowing about many-worlds, or at least independently of this fact, and is using many-worlds to justify his preference.

Where on Earth are you pulling this from? If my memory serves me, I converted to average utilitarianism as a direct result of believing in a Big World.

This argument doesn't seem very strong to me. I could just as well say, "I don't need to worry about a high average quality of life, because the average is fixed, and is as high as it can be in any case. I just want to see as many people in my world as I can, in the worlds that are my responsibility."

You've just violated Egan's Law; your statement does not add up to normality. The quality of life is not, intuitively, "fixed" in the one normal world you once thought you lived in; you care about the future and try to steer it. Quality of life is not independent of your decisions in many-worlds, either.

iwdw:

Why tell readers that their other selves in other worlds are dying of cancer, so they should really think about cryonics, and then go on and make a post like this?

Because some of their future selves, within their present self's control and responsibility, will go on to suffer the same fate.