What an interesting question. It's obvious that you know quite a bit about this. I, having only a few minutes to spare during finals week, do not have the time to learn about things as deeply as you have. However, looking at what you've given me, I can take a shot.
First, a brief statement on the Apocrypha. As stated in D&C Section 91
, the Apocryphal books have a lot of truth in them, but also contain many things "that are not true, which are interpolations by the hands of men." As such, it's important to take many of the things they say with a grain of salt, and, as the Lord instructs, to read with the Spirit, which teaches truth.
The specific teaching of the Book of Enoch that you have brought up is one such interpolation by the hands of men. At this point in the history of the world, only God the Father has an immortal physical body. Even Christ, who had not been born on the earth yet, was a spirit. Thus, the idea that there were beings with God that would be able to leave heaven and marry human (physical) women and produce offspring with them is clearly opposed to LDS doctrine.
The reference you point to in Jude is, however, perfectly true. The first estate is referring to the opportunity to come to earth offered in the premortal life. This estate was forfeited by Lucifer and his followers, and as such they were cast out of heaven. They became, in some sense of the word, fallen angels. The more appropriate term would be fallen spirits. They were cast to the earth and dwell here still.
As far as the LDS viewpoint on the Nephilim...well, that I just don't know. They are mentioned in the Bible, as you said, in Genesis 6
:1-6, but I don't know really how to take the wording of that scripture. In the King James Version, the verse says that the giants were the children of "the sons of God" and "the daughters of men." The Bible, being written in a patriarchal nature, may more highly revere the position of son in relation to God, calling mortal men (as we do now) the sons of God. We also now refer to mortal women as the daughters of God, but it's possible that to demonstrate the lesser importance of the female role the daughters were referred to as the children of men, not of God. How these unions created giants, I'm still not sure.
Leave it be said: it is not possible for a spirit to father a child (as to how this relates to the parenthood of Jesus Christ, see here
(last paragraph on the page)). As the fallen spirits that followed Lucifer were not endowed with physical bodies (indeed, no non-mortal was so endowed until the resurrection of Christ), they did not father children with the women of the earth.