* Having researched Moses' Egyptian heritage and Abraham's journey out of Egypt to the Ur of the Chaldees, how did they impact a religion called Judaism?
Silas gave a good response to this, but I would like to add to it, somewhat. Aside from the historical connection that these figures (actual or mythical) held with the Egyptians, there is also a large section of Judaic thought (as well as christian, though a number of these ideas are beginning to fade as understanding increases) that is derived from Egyptian mythologies. One example concerns the idae of God as Judge. Many people in the past (and even a number today) believe that when you die God will be waiting for you with a scale to measure out all the good and bad deeds of your life, which will determine how you will live eternity. This idea comes from the Egyptian mythologies, which contain a god of the dead who performs this very function. There are a great number of concepts in Judaism and Christianity, similar to this one, that share commonality with Egyptian mythology. The effect it would have would be minimal, since many concepts in Egyptian mythology are already found (though altered over time) in Christianty. What WOULD have effect is telling people about it. This would both bring a new understanding to Christian thought, as well as clarify its validity or invalidity, and bring a more mature perspective of the reality. Back to the example I gave, that is a common belief held by the unphilosophical, and untheological. Those who take the time to study such things understand that this would not be how God would work, but rather, the judgement made in death would simply be a recognition of the state of the soul, and the choice made by the individual, rather than an active condemnation or rewarding by God.
* And, what if Moses and Abraham are merely myths as some say, what does that make Judaism and later Christianity?
If they were myths, then, as most myths held historically, it is probably based very much in reality. The fact that such names as Abraham, or Moses are more than merely names, indicates that those (at least in the era of their telling) would have understood the meaning of those stories in their truest sense, and so the religion that grew around these stories would have been based on the true meaning of them. Because those stories were written and then translated into other languages, much of the meaning of those stories were lost, and only certain aspects focused upon. For example, the cain and abel storie isn't really about two brothers. It's actually about brother sins, that is, two common wickednesses that arise together. The literal meaning of Abel ('b'l in Old Hebrew) is "vanity" (as well as "nothingness," "air"). The Tower of Babel shares this root word, as well as Babylon. Cain (and I've read different heard different meanings) means "jealousy." A truer meaning of the story of Cain and Abel is that after the fall of mankind, these two evils were born within the heart of man. Often times in life, it seems that the vain are the ones that are blessed by God (in Old Testament thought, material wealth was considered a blessing by God, and poverty was considered a curse) by wealth and beauty, etc... With vanity, also comes those who are jealous. As the story illustrates, jealousy can lead to terrible crimes. Such meaning would have been understood by those of the original language, but have been lost in transliteration. As these stories become clearer in this the "Information Age," alot of misconceptions are being done away with. The difficulty is that many people are wary of anything that opposes their beliefs, and it is difficult to both convey the real meaning, and its impact on Christian belief.
* It's widely known that the dying demigod savior, Jesus, was just a myth of a myth of a myth, so where does that leave Judaism and Christianity?
Right now, it is accepted that the story of Jesus has MANY similarities with other myths, both in Judaic scriptures and from other religions. However, what is not clear is whether Jesus Himself was a myth, or otherwise. Something definitely happened around the period of 0 BC/AD that affected a very large movement, which became known as Christianity. What that was is up for debate. I think, though, that it really has little impact on the teachings (at least morally speaking) of Christianity (or at least Catholocism... I know many Christians would say that belief is all one must have, but Catholocism teaches good living as a definite moral base).
* If Paul preached Christianity, when Paul never knew Jesus, where does this impact Christianity?
It is widely accepted that Paul never met Jesus (I assume this is what you mean by "know), and so probably wouldn't really affect the teachings of Christianity very much. It is well known that Paul was a killer of Christians for a time. How Paul came to change is way of acting, is a matter of debate, though it is told that he had a vision of God. A vision can be as simple as a realization, an actualization of a reality in one's own mind. Perhaps Paul has been moved for quite some time by the willingness of the early Christians to hold their beliefs even in the face of death. Perhaps he had heard some teaching spoken by a Christian, and in his travels (which, in those days, provided much thinking time) came to a realization of some truth within the early Christian faith. Obviously one doesn't come to a full understanding of any given religious idea, or philosophical thought in a single moment, it is a given that he, most likely, went to Christians to learn their beliefs. Probably, he likened it to those beliefs he already held, and had discussions with one or more of the Apostles, who either confirmed or denied, or considered the extent of his beliefs as being Christian. I find it unlikely that all of the Christian movement of the time would have been convinced by Paul, when so many had already learned much from Peter, and the other apostles, as well as a number of disciples of Jesus, if his teachings were not true to early Christian thought.
* If Jesus was a Jew and not a Christian, where does this leave Christianity?
The early Christians were considered as a form of Judaism (rebellious albeit), and sometime later became known as Christians (followers of the teachings of Christ). In other words, early Christians would have considered themselves followers of the true teachings of Judaism, and would have considered the Judaic teachings of the time as faulty in light of a better understanding of OT ideas. It is well known by Christians that Jesus was a Jew, he honored Jewish laws, this is shown quite well in the NT. He even said (I believe) that He came "not to undue the Law, but to fulfill it."
* If the virgin birth is an oxymoron, where does that leave all young women having their first baby?
Again, Silas answered this one well, I don't feel a need to comment on it.
* If Jesus was God, and not just a rabbi, there would be no need for Christianity -- Jesus would have been the Jewish Messiah -- but he wasn't. They're still waiting.
And again, Silas answered this one well. It isn't that Jesus wasn't the Jewish Messiah (He indicated, along with the writers of the NT, how He fulfilled the prophesies of the OT), rather, it's just that the Jewsih belief is in a political leader, savior, rather than a spiritual one. There are actually still a number of Jews today that come to believe in Jesus as the Messiah.
* If the Sumerians were a myth, and the Babylonians were a myth, and the Mesopotamians were a myth, and the Mithras were a myth, and the Egyptians were a myth, and the Hebrews were a myth, and Christianity was based on these myths, what is the truth?
While Silas answered this one well, once again, I would also like to add response of a different kind. Myths themselves come out of legends, and legends come out of actual events. How much the myth actually resembles any historical event which they might be based upon is a simply a matter of chance. Each myth emphasizes a specific aspect of any given story, and therefore it is up to the reader to understand that aspect in relation to both the entire story being told, as well as his own life. Furthermore, it is also a matter of understanding the exaggerations or emphases, and what other important aspects of the story exist, and what they mean in relation to reality.
* Does humankind simply need to believe the myth to survive?
No, myths usually aren't about survival. Neither is the story of Jesus. Such stories are usually about happiness, and how it is to be achieved, and how unhappiness is to be avoided (i.e., the Cain and Abel story).
* If there is no god, there can be no savior. Then why would humankind need to create their own savior?
While Silas' answer seems to be on track, it ignores that Christianity teaches that whether saved or otherwise, there is be no destruction of the soul, and EVERYONE will be raised from the dead, not just those who are saved. No, it isn't an escape from death that humans require, it's an escape from unhappiness. This is especially seen by those Christians who believe that all that is required in this life is belief, since then even the pain of guilt is removed. This is also the motivation of Buddhism, and a great many of the world religions. It isn't about escape from death, it's about escape from unhappiness, or the persuit of happiness.