wait, why are we assuming that adding the new people and sharing with them always generates more utility, why are we assuming the amount of utility the people in A lose by sharing with A+ is always exceeded by the amount the people in A+ gain?

Right, going from A+ to B might require increasing the amount of resources available if it has to avoid decreasing total utility, and if it does, then you can't derive the repugnant conclusion as an actual policy recommendation. Although diminishing marginal returns suggests that going from A+ to B usually will not require adding resources, but going from A to A+ will. [Edit: I was about to add a link to a post explaining this in more detail, but then I realized that you wrote it, so I guess you understand that]

Edit2: And you still haven't answered my question. Why N=2?

I was trying to say that a more complex, multifaceted theory of ethics, such as the one I propose, is necessary to avoid various frightful implication of more simplified ethics.

Forget "a naive utilitarian who doesn't ... might ...". If there are a bunch of people whose lives are so terrible that it would almost be better for them to kill them out of mercy, but not quite, and keeping them alive takes a lot of resources that could be very useful to others, I would endorse killing them, and I find that fairly intuitive. Do you disagree?