If the people of Nod are NOT descended from Adam and Eve, then presumably there are descendents of theirs who are not descended from Adam and Eve — if you even believe that that story is anything more than a story. Unless Cain’s wife was the only female in Nod, which I would think is VERY unlikely.
Now, if that is so, that blows a big fat hole in the doctrine of Original Sin, doesn’t it?
Demons? It fits with Cain's character or lack thereof.
Regards,
GtG