Angelic blood isn't evil in itself. Trying to subvert God's plan is the evil. Cain's children aren't listed among the descendants of Adam, so I think it possible Cain's conception may be related to the original sin...
Of course, after the Flood, the angels started doing it again. Exodus tells us about Nephilim in Canaan.
You are correct, they did. If you do the math, how many generations does it take for a population to share common ancestors? If the first batch of hybrids hadn't been eliminated, what are the odds of any being free of any of them being in their lineage by the time Mary was born? For example, it is said that all Europeans descend from Charlemagne.
As to the rest of what you say, certainly possible. It's one of those things I look forward to finding out when I die, as I don't think it can be determined perfectly in this life.
I have enjoyed our conversations here, but it appears that my time on FreeRepublic will soon be drawing to a close.
Farewell and Godspeed.