The place of Numbers in the Torah

Modern biblical scholars believe that the Torah, Genesis, Exodus, Leviticus, Numbers, and Deuteronomy, reached its present form in the post-Exilic period, after 520 BCE, based on preexisting written and oral traditions, as well as contemporary geographical and political realities. According to a long-standing tradition, the five books of the Torah were dictated by God to Moses.  However, after the Christian Reformation of the sixteenth century, modern critical biblical scholarship found that the Pentateuch did not seem to be a unified text like you would expect from a single author like Moses.  As a result, the unique Mosaic authorship of the Torah has been largely rejected by leading biblical scholars since the seventeenth century, as they came to realize that the Torah was involved in a long evolutionary process.  In 1878, the German Julius Wellhausen (1844-1918) published his Geschichte Israels (The History of Israel) that established the four-source theory for the Torah, known as the documentary hypothesis.  The first source, the Jahwist (J) source, was the earliest document, a product of the tenth century BCE and the court of Solomon.  The second source, the Elohist (E) source, was from the ninth century BCE in the northern Kingdom of Israel.  This E was combined with J by an editor to form a JE document.  The Deuteronomist (D) third source, was a product of the seventh century BCE, during the reign of King Josiah (640-609 BCE).  Finally, the Priestly (P) source was a product of the priest-and-temple dominated world of the sixth century BCE.  The final redaction, was when P combined with JED to produce the Torah as we now know it.  Thus, the final redaction of the Torah might have been during the Persian rule between 450–350 BCE.  Most scholars today agree that some form of a P source existed, although its extent, especially its end-point, is uncertain.  The general trend in recent biblical scholarship is to recognize that the final form of the Torah as a literary and ideological unity was based on earlier sources, likely completed during the Persian period.  Thus, the final composition of the Torah came in the fifth century BCE.  Numbers was a Priestly (P) redaction of a non-Priestly original.  Many evangelical biblical scholars prefer to hold on to Mosaic authorship, which can manifest as belief in Moses as the sole author, the substantial author, or a contributor alongside a second anonymous author.  One of the reasons to maintain a Mosaic authorship is that many early Jewish and Christian sources assumed Mosaic authorship.  However, it is plausible that there was a pre-exilic written tradition of passages from Numbers.  What do you know about the origins of the Torah?