I noticed you changed units between the average distance of another you and the average distance of another identical universe. That seems rather pointless. A lightyear is only 16 orders of magnitude larger than a meter, and is lost in rounding compared to 10^115 orders of magnitude.

You mentioned a portion of people. I don't think there's any reason to believe that the universe is this big but still finite, and if it is infinite, there's no way to measure a fraction of people. There are infinity people who's lives are worth living and infinity who's lives are not. If you add it together, the result depends on what order you add them in. Dividing is similarly nonsensical. You can't change the proportion of people who are happy because there is no proportion of people who are happy.

"But on the whole, it looks to me like if we decide to implement a policy of routinely killing off citizens to replace them with happier babies ... We're just setting up the universe to contain the same babies, born predominantly into regions where they lead short lifespans not containing much happiness."

That would mean more happiness. Also, I don't see the problem with short lifespans. My instinct is that you think consciousness ending is bad, but that happens every time you go to sleep, and I don't see you complaining about that.