I would not kill a million humans to arrange for one billion babies to be born, even disregarding the practical considerations you mentioned, and, I suspect, neither would most other people.

It's extremely hard to get away from practical considerations here, and it tends to be hard for people to generalize ethics to things as far removed from practicality as killing a million adults to replace them with a billion babies.

This perspective more or less requires anyone in a position of power to oppose birth control availability, and require mandatory breeding.

It would? While mandatory breeding (via birth control denial or other measures) would make for a lot of people, their lives would be much worse. The reason a person in power opposing birth control and mandating breeding sounds horrible is the same as why I would oppose it: it would suck. No one wants to be forced to have kids.

I also care much more about the total number of people to ever exist (again, weighted by how good their lives are) than the total number to exist at once. Dramatically increasing the number of people alive now, even if you did it in a way that didn't affect average happiness, would probably just make us burn through our current stock of resources faster and not lead to more long-term total people.

I would be about as happy with a human population of one billion as a hundred billion

Now this sounds deeply insane.

(Not as an insult! It just seems horribly scale insensitive.)