Regarding the conflict with the Nephilim, I have no doubt that violent conflict marked the interactions between peoples, whether racially, culturally, or merely distinct from living in a different village, from time to time. Certainly history is written by the victors, and their vanquished foes always claimed to have been evil, whether they actually were or not.
Regarding Eve, the Toba eruption was a devastating cataclysm that killed the majority of people on Earth about 75kya, and it is easy to believe that genetic drift caused a single mitochondrial line to alone survive afterwards. I reckon this artifact of the vagaries of evolution underlay the acceptance of the Out of Africa Theory Tattersall proposed, despite much evidence contrary.
However, many human species survived Toba, and it is now impossible to ignore the heterotic hybrid nature of extant human populations, which utterly refutes the Out of Africa Theory.