I am not sure how this helps anything. Previously you’ve been defining humans theologically and not scientifically, and now you want to define fallen angels as “essentially biologically humans from a scientific point of view”. I’m not sure how many scientists would agree with that assessment, but from an ancient perspective fallen angels were distinctly non-human.
We need to remember that the author(s) of the Book of the Watchers was not trying to solve the same problem you are addressing. They weren’t providing an explanation for an alternative source of genetic material for Adam’s lineage, or trying to solve the problem of where Cain’s wife came from, they were providing an explanation for the identity of the “Watchers” of Daniel 4, and the “sons of God” of Genesis 6, which they identified as non-human fallen angels. Not only that, but there’s clear evidence in the Book of the Watchers that this mixing was regarded as a transgression by virtue of being unnatural.
I think that’s like saying that since Paul would have known of the Books of Enoch, but didn’t speak against angels having sex with human women, then he had no problem with angels having sex with human women. Paul was raised and taught first in Cilicia and then in Jerusalem. It’s extremely uncertain that he would have known of the Books of Enoch, given that the only certain first century witness to them is in the Aramaic texts of the Qumran collection; these were not mainstream texts, they were fringe. Making positive statements on the basis of what Paul didn’t say about works he may or may not have known, isn’t exactly a sure footing. What we do know is that there’s no evidence Paul held to the Enochian harmartiology, angeology, and satanology, which makes it unlikely that he read the Books of Enoch, and even less likely that he approved of them.
In the entire New Testament there’s only one positively suggested reference to the Books of Enoch, and that’s in Jude (1 and 2 Peter are a “maybe”). If Jude is referring to the Watchers when he speaks of the “angels who left their first estate”, then he is most certainly condemning their actions, because he uses them as an example of the sin of “going after strange flesh”, which is about as clear a condemnation of inter-breeding as you could get.