Greetings, those are good questions...
the one about Cain's wife has been asked of me quite often, he married one of his many, many sisters, and there is actually a pretty good overview on it here...
http://www.answersingenesis.org/arti...was-cains-wife

and another one here:
http://www.apologeticspress.org/articles/2181

Like Winman I can't answer all of these right now, but regarding no. 5 and no. 6, I will refer you to my extensive thread at the link below which contains the answers you seek and a lot more about the sons of God and the daughters of men... the entire thread is pretty good reading, but if you're in a hurry you will find that posts no. 5 and 6 on that thread lay down some pretty good timber on the subject.
by the way, what is the meaning of the title to your thread?

Giants in the Bible - Nephilim
http://av1611.com/forums/showthread.php?t=896