SCHOLARLY PERSPECTIVE:
The scholarly perspective is that there were various traditions and different texts that were knitted together at some point. What fragments came from what original text, and when they were knitted together, is a large topic (and there is considerable disagreement, even within the scholarly community). One theory: the original stories were oral traditions, written down around either 1250 to 1000 BC (either time of Moses or time of David), and then combined into one narrative after the destruction of the Northern Kingdom (so before 700 BC).
Thus, the two creation narratives are two different oral traditions, one placed after the other, to create a single national (Israelite) text.
The identity of the Nephilim, Sons of God, etc were well known in the oral tradition, but lost in the transition over centuries from oral to written versions.
OK, now draw a thick line.
TRADITIONAL APPROACH:
The traditional approach (as opposed to the scholarly approach) holds that the text of Genesis was written down by Moses, dictated by God at Mt Sinai (around 1250 BC). The identity of the Nephilim or the Sons of God etc was possibly known by oral tradition to the Israelites at that time, but we have lost that knowledge. There has been considerable speculation over the centuries, but there is no definitive answer.
The second creation story starts over because it tells the story from a different perspective. The first version is about the universe – stars and earth and fish and birds and such. The second version is about human kind, and puts them at the center of creation. The two different versions are designed to teach different moral lessons, and both are correct.
THE OTHER QUESTIONS:
On your other questions, like who married Cain or Seth, the text is silent. Whether the Author is human or divine, this is information that was not considered important enough to pass on.
Why stop there? The text doesn’t say how tall Cain was or what colour his hair was. Doesn’t even describe his skin colour. The text doesn’t indicate the weather on the day that Cain killed Abel, or describe the terrain. Doesn’t describe the clothes they were wearing. Doesn’t indicate how many blows were needed for the deed. The text is scanty. It is focused on the horror of murder (“spilling blood”) which it exemplifies though fratricide.
Why such a big deal about elements that the text DOESN’T address?
In the Odyssey, Homer doesn’t tell us what song the sirens sang. Yet no one picks at Homer on account of this “glaring omission.” But every Tom, Dick, and Roy fret about who Cain married… like the absence of that information somehow invalidates the text.
[/sermon] Don’t worry about what the text doesn’t say, worry about what it DOES say. It says stuff about being your brother’s keeper, and its says to deal charitably and with lovingkindness with each other. It’s a moral guide, it’s not a treatise in geology or heredity or biology or paleontology. And that’s true, regardless of authorship. [/sermon]