A huge thing to notice when reading my reply in the following is to notice how I attempt to connect Genesis chapter 5 to Genesis 6:1-2.
I agree with the different usages of 'adam. It is clear that verse 1 is used for the entirety of mankind. In verse 2 you have the entirety of mankind split into 2 groups. God's chosen line verse the non-chosen line (offspring of Cain and Abel). Can you please explain why the author (Moses) would not split mankind into God's chosen? I find that to be a great explanation of the context of Genesis. Is not the entire book about God's bringing about His promised Messiah through a chosen people? Notice that Moses listed the descendants of Adam that build up to the corruption of mankind (chapter 5-6). Noah was following God's chosen descendants that Luke recorded in chapter 3 that end up leading to Jesus. Look:
Genesis 5:
v.3- Adam
v.3- Seth
v.6- Enosh
v.9- Kenan (or Cainan)
v.12- Mahalalel
v. 15- Jared
v. 18- Enoch
v. 21- Methuselah
v. 25- Lamech
v. 26- Noah
Notice that only the promised line "sons of God" (or people belonging to God's promised line) are mentioned and followed in chapter 5. Specifically in the chosen line, only righteous descendants who were chosen to carry the seed of Eve were mentioned and all the other sons of God's promised line are labeled "other children."
Now compare with Luke 3:36b-38.
Context is everything and it is almost definite that the "sons of God" are the chosen or elected lineage from Adam to Noah. Now the rest of mankind apart from the descendants of Seth "sons of God" are simply the descendants of Cain or "daughters of man", that is to say "men of the flesh" not "men of the promise" . Yet at that time all descendants from both lineages are living in sin and intermingling. All but Noah and 7 relatives were righteous. Later God preserved His promise in Gen. 3:15 through Noah's son Shem (Luke 3:36) and so on. This is the overarching context of Genesis and the Old Testament- Redemption. This is why I believe context is the greatest evidence that "sons of God" refers to Sethites or (God's chosen lineage, Luke 3) rather than angels or Nephilim.
Lastly the phrase "sons of God" is foreign to Mosaic writing and is not found in any of the 5 books. It's best explanation isn't from pulling terms from the book of Job or the prophets. The best is to understand it in the context of Genesis and from the New Testament teaching, other historical writing from the day. I will be interested to see what outside sources say about Genesis such as the book of Enoch you quoted.
This is interesting...