Excellent George!
I appreciate the @Swamidass Model as well, but I think it contains an oddity that deserves being removed to get a coherent formulation of Original Sin’s transmission.
According to the model (I quote you):
If one keeps to the Sanctity of Marriage one has to accept that the offspring of Adam & Eve mingled with Image Bearers. Therefore the “hominids” you refer to became Image Bearers before mingling with Adam & Eve’s offspring.
So far there is no problem since Genesis itself accounts for this possibility in the episode of the “sons of God” (Genesis 6:2-4): God directly transformed these “hominids” into Image Bearers, that is, created new human persons (Image Bearers) without collaboration of other Image Bearers as parents.
But now the question arises:
Did these Image Bearers (God directly made “after Expulsion”) share “the state of Original Sin” (“Lack of Original Grace”)?
If NO, then you are acknowledging on Earth two radical different groups of people: One In-need-of-Redemption, “the offspring of Adam & Eve”, and another Not-in-need-of-Redemption, the Image Bearers made “after Expulsion”.
If YES, then you are acknowledging that each Image Bearer coming into existence “after Expulsion” shared “the state of Original Sin” (“Lack of Original Grace”) since the very moment they came into existence as Image Bearers. This is my proposal Transmission at Generation.
The preceding reasoning applies to @Kathryn_Applegate’s proposal of “Adam as Representative” as well.
In summary, it is very promising that we have now three models assuming Adam as a “real Person” and Original Sin as a “real Transgression” in History, and it is worth discussing seriously these proposals to get a coherent explanation of the origins of Humanity from the perspective of Jesus Christ’s Redemption.