With respect to your comments about the Nephilim. I am very familiar with that school of thought, and it is erroneous, along with the uncannonized lost books of the Bible you referred to.
There are no such classification of angels outside of the Serephims that are half creature and man like you described. There are only 3 classifications of angels, and none of them are sons of God as you state the Nephilim are.
Those translations were at the behest of the translators who were not filled, or anointed, with the Spirit of the Lord... as written in Job about the sons of God appearing before him.
Where the error begins is with the KJV which is rooted in the Latin Vulgate. In other words, the term "sons of god" is erroneous from the start when referring to angelic being.
They are all servants sent forth to minister to those who would be heirs to salvation. it is obvious from the root of the Hebrew word used it is talking about angels and not men. Only men can be called sons of God, who walk by faith and not by sight.
There is a greater (lengthy) explanation with reference to this, but suffice it to say that the doctrine of angels being called sons of God was all due to the fall, or curse of the fall, through Adam and Eve and is a precept of men.
The Nephilim were a group of angels that rebelled after the original fall of Lucifer (who has 6 abodes) and who took a third of the host with him.
They (the third host) are here on earth, whereas the Nephilim, because they rebelled afterward, were put into chains of darkness. Be a good scribe and look it up yourself.(ref. scriptures; Isa.14:1..; Ezek.28:12; Rev.12:1)
Jude 6 talks about these angels due to the fact if he was talking about the third host, then there would have been no devils for Jesus to cast out of people in his day..and in mine..and yours.
What sayest thou?