Yes, agreed with all this.

And yes, like most people, I don't have a coherent understanding of how to aggregate intersubjective utility but I certainly don't aggregate it in ways that cause me to embrace the Repugnant Conclusion. (By contrast, on consideration I do seem to embrace Utility Monsters, distasteful as the prospect feels on its face.)

Your choice of how to aggregate utility also affects whether it is good to trade off utility among already existing people just like it affects whether it is good to create new people.

Well, not "just like." That is, I might have a mechanism for aggregating utility that treats N existing people in other countries differently from N people who don't exist, and makes different tradeoffs for the two cases. But, yes, those are both examples of tradeoffs which a utility-aggregating mechanism affects.