I agree with your comment about St. Paul’s theology underlying the endeavor to defend monogenism (or monogenesis, as some prefer to call it). I realize that if you were arguing from Genesis alone, you could make a good case that Adam is meant to represent Everyman. But if Jesus Christ is the second Adam, as St. Paul argues, and if Adam is not a single individual, then why should Jesus Christ be? The symmetry of the metaphor appears to require that if the second Adam is a historical person (as Christians believe), the first Adam should be too.
We seem to be in agreement that if there was a historical Adam, he must have lived over a million years ago. Dr. Ann Gauger identifies him with the 2-million-year-old progenitor of Homo erectus, while Dr. Dennis Bonnette suggests that he was the first specimen of Heidelberg man, around one million years ago. That saves Adam, but as I’ve argued in a recent post on The Skeptical Zone, the point at which modern human behavior appeared seems to be around 100,000 years ago, not one or two million. Homo erectus and Heidelberg man possessed a kind of rationality, but probably not language or symbolism. See here for more: http://theskepticalzone.com/wp/envs-shockingly-bad-argument-that-humans-are-products-of-intelligent-design/ . So if Adam was the first Homo erectus or Heidelberg man, he was a lot dimmer than we are. It is hard to see him as our equal.
I have also been prepared to entertain the notion that maybe Adam was the head of a tribe, whose members agreed to let him decide the spiritual fate of the human race. If so, then perhaps a bottleneck of a few hundred individuals would be theologically acceptable. But if the Fall took place within the last million years, then from what Professor Venema has written, it seems that the lowest we can go is a few thousand individuals - which is far too many for a single tribe.
Other authors (such as Kenneth Kemp) have proposed the notion of ensouled humans interbreeding with virtually identical hominins, thereby accounting for the large bottleneck. But the notion that two individuals might be genetic conspecifics, with one possessing a rational soul and the other lacking it, makes no theological sense. And what would happen to their offspring? It all sounds very ad hoc to me.
Finally, some have proposed a Neolithic Adam. That certainly harmonizes with the Biblical chronology and the data in Genesis 4, but the idea that humans were not aware of God until a few thousand years ago makes no sense. Surely Cro-Magnon man worshiped something. And if he did, then why should a choice made by one of his Neolithic descendants have any negative implications for the human race as a whole?
So it seems we have a bad set of options. Which is the least bad? Search me.