Fake Iosefka bit is the real confusing one. Forgive me if I get tangential with this one. I used to think that Iosefka/choir could convert people into both Celestial beings and larvae surgically, grown ups would become usual bubbleheads and infants would become larvae. But then Arianna gives birth to a larvae.. which throws the whole thing off. So perhaps surgically humans can only be converted into bubbleheads and the larvae are a result of a proper birth.
The thing is, Ebrietas can't birth them. The whole R/K selection thing that Miyazaki talked about with the Great Ones doesn't let them have more than one child apparently, which they lose eventually. That's why they look for surrogates. So where did all these larvae come from? I might be completely offbase but it could be other Yharnamite women through Oedon just like it was with Arianna. The choir kept all these larvae around to please Ebrietas or for other reasons.
Then there's another thing which throws what I said above out the window, Celestial Emissary can summon more bubbleheads out of nowhere. And like you said Rom can summon her Spiders but she's not giving birth to them, so where are they coming from? ¯\_(ツ
_/¯