Lightbrite: Is a first born man who was born through a cesarean section and/or to a mother who had a previous miscarriage still considered a first born in this case, if it is something regulated to first borns?
Sorry, I just saw your question now. Interesting question. I haven’t seen anything on the subject of which firstborns will do the Avoidah.
If the source to the idea that the avoidah will return to firstborns is the Midrash כי לי כל בכור, then we are exclusively talking about פטר רחם, which refers to a firstborn who opened the womb of his mother. Born through a c-section, or after a miscarriage (in most cases) is not considered a firstborn for Pidyon HaBen, and it probably has the same Halachos.
I don’t think it’s much of a proof, but in Hilchos Bircas Kohanim, the Leviim must wash the hands of the Kohanim prior to the blessing, and if there are no Leviim present the Bechorim will wash instead. And there the Poskim explicitly state that only a Bechoir Peter Rechem (Min HaEm, not Min HaAv) can do it. See Magen Avraham here. I wonder if this is connected at all.