Well, I am by no means any where near as scholarly as you, so I take a bit of a different approach. Where you have great knowledge and read books and and compare, historical people and places, I look at words and try to relate to them.
Gen 6:1-5 "When human beings began to increase in number on the earth and daughters were born to them, 2 the sons of God saw that the daughters of humans were beautiful, and they married any of them they chose. 3 Then the Lord said, “My Spirit will not contend with[a] humans forever, for they are mortal[b]; their days will be a hundred and twenty years.”
4 The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of humans and had children by them. They were the heroes of old, men of renown.
5 The Lord saw how great the wickedness of the human race had become on the earth, and that every inclination of the thoughts of the human heart was only evil all the time. "
Because in vs2 it contrasts sons of God, to humans, that leads me to believe the sons of God aren’t humans. So option 3,4,5,6 is ruled out.
It also speak in vs 4 of Nephilim, which we don’t know exactly what they are either, but they came from sons of God, not humans, or it would have said humans had human offspring.
I rule out option 2, because there are no “Gods” in the Bible that I am aware of. Just one God, and it mentions gods or god, with a lower case “g”. Like John 10:30 (referring to Psa 82:6) In the lower case “g” I think it means beings of authority/power, who are immediately contrasted with saying how these “gods” are mere mortal. Though all men in a power position are placed there by God, so in a sense they are gods as they are an extension of God.
That leads me to believe option 1.
Vs 5 says how “every inclination of the thoughts of the human heart was only evil all the time”. And in vs 2, it says “any of them they chose” They were self serving, not serving others of God, any/every self serving motivation is evil. I don’t think it was just the violence they may or may not have spread with their metalworking knowledge. It was the self serving evil heats of men, with no regard for the daughters of humans or humans, they did what they wanted. Vs 11 says “corrupt AND full of violence”, not just violent.
I don’t think it has become any more corrupt or violent then, than it is now. I don’t think God was surprised by the corruption, He knew Jesus would be the Ark of Redemption of mankind, and wanted to foreshadow a wooden Ark of redemption for mankind.
It was only a few years after Noah, that corrupted man was already back to self -serving, trying to make themselves a name for themselves, trying to make themselves higher than God or able to reach God by their own efforts.
I guess this is a ‘step up’ from blatantly rejecting God’s creatures and treating them violently, but it shows that even in "being moral’ it was still not right. I mean building a massive building would take great love, teamwork and cooperation, some might call that a utopia. But we will never be able to reach God or be image bearers of God by our own strength.
But He didn’t destroy the earth again with a flood, because He was busy having the new ‘Noah’ (Abram through Joseph) ‘building’ up the Ark of Redemption of Jesus.