Interesting stuff here! I'm intrigued by the concept of archaic sapiens forms comingling with pre-european neanderthals, slipping them the sapienesque mitochondrial DNA.. Then the part about smaller population size leading up to the accumulation of less favorable genes which then allowed the sapiens x chromosome to be so easily ushered in to the Neanderthal genome. They pondered what benefits were had through this substitution, and I can't help but wonder if it could be something so simple or basic, what with these two "substitutions", as those Neanderthals with the "upgrade" looking just a bit more sapiens than the neighbors, and there by tolerated by the actual sapiens longer, like they were less alien looking so they didn't have to die, right off.
This does also somewhat support the idea that we sexed the other hominids out of existence....