I for my part am convinced that in the two genealogies of Genesis 1-11 we have the legendary origins of two different people groups, both of whom had an important part to play in the history of Yahwism. Genesis 4 I am convinced was the origin story for the Qenite, or Midianite people who lived in the deserts around Sinai, and were probably the original worshippers of YHWH (See Deut 33:2, Judges 5:4 and Habakkuk 3:3). This explains why Cain (whose name is cognate of the Qenites) is given a mark of divine protection, despite being a murderer, it also explains why Cain is the first to worship YHWH. Genesis 2-4, I hold is very old, possibly the oldest part of the Bible.
Then later in the narrative of Genesis 4 we hear that the Sethites were the first to worship YHWH, which contradicts the earlier story, and is probably an interpolation. I follow Amar Annus in linking Seth to the Sutean people. Genesis 1 and 5 are almost certainly exillic or post-exillic, but I would suggest that Seth was an earlier figure in Hebrew folklore, likely an ancestor figure, with the Suteans being the ancestors of the Israelites, as well as the other Hebrew kingdoms.
Just my insight into the early chapters of Genesis.