Moses was not the author of the Torah - God was. He dictated the Torah to Moses on Mt. Sinai. Most of the "problems" found in the Torah by those who promote the Documentary Hypotheses can be explained if you look at God as the author.The end of Genesis is a convenient cutoff for the history/mythology line, I suppose, but there's one problem. Apparently Moses didn't get that memo:
For instance, one of their main complaints is that Moses is described as a humble man and a humble man wouldn't describe himself as such*. However, if you look at God as the author, He can describe Moses as a humble man.
Also, the reason for the different versions of the same story in Genesis, such as the creation story is to tell people not to take these stories literally. None of the stories in Genesis should be taken literally.
Genesis 50:25 - And Joseph took an oath of the children of Israel, saying, God will surely visit you, and ye shall carry up my bones from hence.We don't have an issue to deal with because Moses in Exodus repeats the words of Joseph from Genesis as if they were to be taken literally. Instead we have a bag of old bones to deal with, a physical artifact that bridges the chasm from mythology to Moses. If no story from Genesis is to be taken literally, whose bones are these? And why would Moses assert real bones belonged to a person he knew to be mythical**? If they are, as Moses states, Joseph's bones, then there must be a Joseph and he must have died in Egypt, and he must have come from the place to which Moses is heading.
Exodus 13:19 - And Moses took the bones of Joseph with him: for [Joseph] had straitly sworn the children of Israel, saying, God will surely visit you; and ye shall carry up my bones away hence with you.
In short, to take Exodus historically we have to take at least part of Genesis the same way, or we must insist on the non-literalness of an unremarkable story that is the most straightforward explanation of the physical existence of Joseph's bones in Egypt***. But if we have a real Joseph, then we have established the historicity of 40% of Genesis (the last 20 chapters). If we have 3 more historical generations - Joseph must've come from somewhere, after all - then we have established the historicity of more than 70% of it (the last 36 chapters).
So from that point, where can we make a non-subjective cutoff from history to mythology? I don't have an answer to that question, but I'm pretty sure it's more complex than just picking the end of Genesis, convenient as that might seem at first glance.
* The easy solution (and I think the correct one) to that particular conundrum is that one of the later copiers of Moses' writings inserted that as explanatory material. That doesn't take JEDP at all, nor should it trouble those who believe Moses wrote the Pentateuch.
** Or didn't know to be mythical. To assert that God first revealed Genesis at Mount Sinai is to assert that Moses left Egypt after quoting the mythical words of a mythical man that had not been revealed yet. That would mean that God created for Moses a mythology of Joseph that was in synch with what Moses had made up - he must have made it up if Joseph did not exist and God had not revealed his myth - for the Hebrews previously. Sorry, that makes very little sense to me.
*** Most JEDP scholars understand this perfectly well, which is why they deny the historicity of any of it. That option is not open to J. Field because he has already posited a real Moses, which presumes an historical Exodus.