Which specific scroll are you talking about? Dead Sea Scrolls and (this sounds like the book of Jubilees or Enoch with Samael in there) Enoch and Jubilees were all written way after the Bible. It is they who are changed from the original. "I have gotten a man from the lord" is in the textus receptus, the Masoretic text. The Jewish scholars of 800 AD think that is what the original said, and they were very careful.
Cain and Abel are not listed with the generations of Adam. Not the sons Towldoth, not beni. Cain and his line perished at the flood; they are hardly generations if they did not survive very long. I agree that the statement "Adam knew his wife and she bare Cain is not as definite a proof as is usually assumed, but the same phrase is used again throughout the OT for conceiving a child, so I see no reason to question the tradition that Eve's first child was from Adam. Abel is called his brother, not his half-brother, so they had the same two parents.
I agree the whole thing is tied with the angels rebellion, but I am not certain that the way you are processing the connection is as good as the standard story of the nephilim. Certainly, angels (later or then fallen) were involved with the birth of Cain and Abel, but I do not think they were the parents.
When God created Adam in his own image, he commanded the angels to serve man.
This command did not sit well with the angels. They were powerful, intelligent, and had been around for a very long time. It must have felt like a put down to them. I/3 of them rebelled. God cast them down as angelic, human like animals. They were close to human, but more of the animal tree.
Satan came up with a plan to get Adam and Eve to fall out of favor with God, then he and the rest of the angels would not have to serve man.
Remember in Genesis 3: 14 to 16? God is talking to Satan like he is an animal. God is also talking about the seed Satan has? God even calls Satan a murder. Who did he murder?
Animals always aged, and died. This is how Adam and Eve knew what death was. When Satan injected animal seed into Eve, That seed mixed with human seed, is why we die. Satan had murdered the whole unborn human race.
Genesis 3: 14 is what the whole Bible is about. When Jesus came as a human, he was perfect…without animal seed. When Jesus died, the seed within him was a perfect, unborn human race. That unborn human race is what was sacrificed for our race. That’s why Jesus only had to be dead for three days.