Methuselah did not drown during the Flood. He died seven days before it started.
There was also a son of Qayin named Chanokh, after whom the first city was named.
So far as I know, there are two schools of thought on the "sons of G-d" and the "daughters of man." One is that it refers to intermarriage between the male descendants of Qayin (who were evil) with the female descendants of Shet (who were faithful).
Now, as for the question about fallen angels, I once asked that very question. I was puzzled because, according to traditional Jewish teaching, HaSatan is not a "fallen angel" but merely another of G-d's employees doing his job. Here is what I was told:
Two specific angels (I was given their names but do no remember them, unfortunately) asked HaShem why He didn't destroy mankind for his sinfulness. HaShem responded, "it is easy for you, being angels, to take a harsh view of man. But if you were flesh and blood you would also sin." The two angels immediately denied this, saying that they would never do such a thing. HaShem asked them "would you like to see? Would you like Me to give you bodies of flesh like man?" They said yes, and they would still resist temptation. HaShem granted their wish . . . but they were unable, in bodies of flesh, to resist temptation. So the "men of renown" were the descendants of those two enfleshed angels.
The moral of the story: no one can resist a babe!
BTW, I am now listening to Georgian language radio online. It isn't as pretty to listen to as it is to look at.
“So the “men of renown” were the descendants of those two enfleshed angels.”
Interesting take. How does that theory explain the post-flood giants though? Did those two angels escape wrath long enough to fornicate some more after the waters receded?