Hi again! I'm not usually online much on the weekend--sorry it took so long to get back to you.
01. Why was Lilith not included in the Bible when she's included in the original Jewish myth?
That would be because Lilith is part of set of folk stories/fairy tales invented after the Genesis oral tradition began. Lilith, as a literary character, appears around 4,000 BC; Genesis dates from 6,000 BC at the latest (recently archaeologists have started to tentatively date it from around 10,000 BC).
Be careful not to fall for the faulty exegesis that places her into the Bible text... that's a well-known fallacy perpetuated by the kinds of "scholars" who thought the Da Vinci Code finally put to rest all their questions about Christianity.
Remember that Jewish mythologies and folk tales have no bearing on Christian Truth*. The Bible and its traditional interpretation are what you need to accept if you're to become a Christian; there's no hidden meaning in the text... there are stories that are hard for individuals to understand, but the Church has already had the Truth revealed to Her by God through Jesus Christ and His Apostles, and in the final analysis, Christians need to submit to that Truth.
I say this because I don't want to mislead you about what it means to be a Christian, that's all. An open mind is fine; reading widely is interesting and good for the brain; but canon is canon, and if you're going to use Scripture to make decisions that effect eternity, you need to stick to the canonical writings. At least, that's if you decide to become a Xian.
*I'd even say they have no bearing on
Jewish truth. From what I understand, no rabbi would teach that Lilith was Adam's first wife, that they begat demons together, etc. etc. Perhaps the rabbi might, at the most, use the folk tales of Lilith to illustrate something... like we would use a Western fairy tale perhaps...Maybe you should contact your local shul and ask the rabbi to discuss this with you?
02. How do Christians account for all similarities between the Sumerian myths and the stories of the Bible (particularly Genesis)?
See my previous post: there are grains of Truth in all human traditions, since human beings are made in the image of God. And what is recounted in Genesis did happen (caveat: Genesis is not an historical document, it's a narrative recounting historical events), so of course its contents crop up in other mythologies.
Note also, as I said above; it looks like Genesis and the rest of the Pentateuch may be the oldest surviving oral tradition on earth, possibly 12,000 years old (10,000 BC). Sumerian civilization is younger than that--8,000 to 6,000 years old (6,000-4,000 BC). = Genesis FTW.
03. Do Christians, generally speaking, take the Bible literally or see it more as metaphorical (except for maybe the gospels)?
It depends what you mean by Christians. And, to an extent, what you mean by "literally".
Most Christians will tell you they take the Bible literally. Different Xians mean this differently though. Some deny the presence of metaphor in the text; others believe that parts of the Bible are metaphorical, because the writer was
speaking metaphorically about a profound truth. Some Xians would regards that bolded statement as heretical...
The EO and RCC don't believe the Bible should be taken literally word for word; both Churches teach that metaphor is obviously present in Scripture. (So when Jesus says "I am the door" he doesn't mean he's made of wood, and has a handle, etc.).
Scripture is not, to the EO at least, an historical or legal document; it is God's revelation to His Creation. It contains stories, allegories, and metaphors; it also contains history. The RCC believes something very similar as far as I know.
We also believe that the Holy Spirit guides the Church in interpreting Scripture (i.e., on discerning which parts of the Bible should be taken completely literally, how NT revelation affects our reading of the OT, how present-day application of doctrine differs from the past, etc.), and that those who rest on their own, individual understanding will end up in trouble. (Proverbs 12:1 comes to mind.)
Lutherans, Presbyterians, and Anglicans/Methodists are all pretty similar to the EO and RCC on many points when it comes to Scripture, although they differ on the position of Church Tradition as a lens for the reading of Scripture.
Once you stray into Evangelical, Charismatic, Pentecostal, and fringe group territory, that's when things will vary wildly (if you don't speak in tongues you aren't saved; if you don't believe in a 6-day creation or that Saturday is the only day for church attendance you aren't a Xian; polygyny is OK if it's your cultural norm; baptism is optional; the Eucharist is purely for rememberance; homosexuals should be barred from Church; etc. etc. The list is very long.)
If you have questions on individual verses, I strongly advise you to contact your priest about them. Don't rely on the internet or on your university reading; Christian interpretation is not the same as secular interpretation. We don't see the Bible as an archaeological document, but as the revealed Truth of God. You can ask us about parts of the Bible if you like, but it would be better to talk face-to-face to a priest.
04. What does the church teach- if anything- about the Book of Enoch & The Watchers, esp. in light of the Dead Sea Scrolls?
The Ethiopian Orthodox Church regards the Book of Enoch as part of the Bible. The RCC and Eastern Orthodox Church regards it as deuterocanonical, and very interesting

. It is quoted in the book of Jude, so it is worth being interested in. As far as I know, Judaism generally holds it as spurious; some ECFs have said this is because the book contains prophecies of Jesus Christ.
05. What makes Christians nowadays believe that the serpent in the Garden of Eden was Satan when it doesn't mention Satan and that was a much later interpretation?
For a number of reasons:
1. Most importantly, because Revelation 20:22 reveals it: "He seized the dragon,
that ancient serpent, who is the devil, or Satan, and bound him for a thousand years". I know you are very interested in alternative readings of the Bible, but Christians hold to a Christian interpretation of the Bible; this verse has always referred, in Xian thought, to Satan and the Edenic serpent. You are free to read it differently, but that will be a secular reading.
2. Because Satan is the Adversary, the one who pulls us away from obedience to God, "a murderer from the beginning", and the serpent is all of these things too.
3. Also: what the Jews believed then =/= what Christians believe today. Jewish and Christian cosmology and theodicy are very, very different.
I really hope this helps a bit! It's always interesting to answer your questions, 23rd. I hope you're having a great Monday!