Exactly how Original Sin arose historically is a secondary issue that admits of some flexibility.
I do not agree with "how original sin arose historically is a secondary issue that admits of some flexibility. I don’t think St Paul thinks it is a secondary issue and he is clear on how original sin with its consequence of death arose, namely, the first man Adam and his sin. Nor would I say the Church thinks that it is a secondary issue. Original sin, as the CCC teaches, fundamentally answers the question “from whence does evil come from” and the present state of affairs in the world.
“The account of the fall in Genesis 3 uses figurative language, but affirms a primeval event, a deed that took place at the beginning of the history of man. Revelation gives us the certainty of faith that the whole of human history is marked by the original fault freely committed by our first parents.” (CCC#265)
Our first parents are none other than Adam and Eve, the first couple. "Have you not read that he who made them from the beginning made them male and female…? (Matt. 19: 4).
(#375). The Church, interpreting the symbolism of biblical language in an authentic way, in the light of the New Testament and Tradition, teaches that our first parents, Adam and Eve, were constituted in an original “state of holiness and justice”. This grace of original holiness was “to share in. . .divine life”.
(CCC#376) By the radiance of this grace all dimensions of man’s life were confirmed. As long as he remained in the divine intimacy, man would not have to suffer or die. The inner harmony of the human person, the harmony between man and woman, and finally the harmony between the first couple and all creation, comprised the state called “original justice”.
(CCC#383) “God did not create man a solitary being. From the beginning, “male and female he created them” (⇒ Gen 1:27). This partnership of man and woman constitutes the first form of communion between persons” (GS 12 # 4).
How original sin arose historically is very clear from the teaching of the Church and Holy Scripture (cf. also the Council of Trent). It arose from the fault freely committed by our first parents, Adam and Eve, the first man and first woman, and it is transmitted by propagation or descent from Adam and Eve, not from Adam and some beast.