In re-reading your post, I wonder if you are "tracking" where @Swamidass is investigating:
1) Using genealogical paradigms, it has been shown that population genetics can be compared to a a flower.... a long stem, with just a few contributors from the distant past, which then deploys laterally like the blossoming of a rose, where a dramatically larger number of contributors from more recent timelines are present in the population.
2) If the YEC's need the special creation of a mated pair, 4000 years ago, anyone who believes that God works miracles could propose that such a pair was created, as long as it acknowledges that they pair was created in the midst of a much larger population of genetically similar humans - - a population that is the natural result of hundreds of thousands and millions of years of mammalian evolution.
3) Computer-run scenarios, using very pessemistic assumptions on human travel, have repeatedly shown that well within 4000 years, a group of various mated pairs can become the universal ancestors of every person in the world now living. Naturally, there are also mated pairings that hit dead-ends, and do not survive into the present age.
4) The Limits of Such a Mixed Scenario?
A. Yep, the idea that Adam and Eve are the only universal ancestor would have to be given up.
B. But, on the flip side, we have the solution to the current diversity we see in the modern human genome. It doesn't come from Adam & Eve, it comes from the non-Adamic population they joined, a genetic diversity created over hundreds if not thousands of generations.
C. Adam & Eve get to experience special creation 4000 years ago.
D. But the mechanism for "the need for redemption" has to be genealogically sustained, rather than genetically sustained, because there would be no support for the idea that all of current humanity is genetically derived from Adam & Eve.
E. If we agree that "the soul" is what needs redemption, instead of the body, then whatever "Federal Headship" might mean under the current YEC assumptions could still hold true in this mixed scenario.
(i) Does God deliver a "soul-that-needs-redemption" to each new person?
(ii) Or is there some sort of psychological "contagion" that each new generation obtains from the prior one?
See, it doesn't matter whether it is (i), (ii) or even some other model.... it still works in this mixed model of Adam & Eve experiencing special creation 4000 years ago, and joining a much larger population of humans not related to Adams, and with or without, any special powers of moral observation that we would currently expect from the descendants of Adam & Eve.
Thoughts, @Jay313 ?
@Jonathan_Burke, you are always my toughest audience. What are your thoughts on how I have parsed out the dynamics?