Igor Lipovsky in his book, Early Israelites, speaks of the “compilers” of the stories in Genesis. He does not have a documentary theory in mind. As best I can tell, he thinks that the southern and northern tribes had oral stories (he usually calls them legends) passed down for several centuries. The compilers worked directly with those oral traditions.
His thesis about two peoples having one history requires that at some point the oral histories about Abraham, Isaac and Jacob were combined and written down. There are a few traces left of the separate histories of Israel and Judah (e.g. Psalm 77:15), but mostly the compilers have succeeded in obliterating the separate histories.
When did this happen? He says it has to be after the turn of the twelfth century BCE. This is because you can see how the compilers have read early Iron Age realities back into the middle Bronze Age. For instance, Abraham travels from “Ur of the Chaldeans” (Genesis 11:31), but Chaldeans did not get to the area of Ur until around the 12th century.
A similar example is calling the people of Gaza Philistines (Genesis 21:32 and Genesis 26:14 ff.). The Philistines only settled the Gaza area after the 12th century.
A more complicated case is the mention of Hittites in Genesis (e.g. 23:10). If they are the Hittites from the area we call Turkey who fought Ramses II in the battle of Kedesh, then they wouldn’t have been in southern Palestine in the time of Abraham. However, after the Sea Peoples caused their downfall at the end of the Bronze Age and the decline of Egyptian power in Canaan, some of them seem to have migrated south.
Also Genesis shows the Patriarchs traveling by camel. This is probably something else that the compilers read back into past as it was much more common in the early Iron Age.
So Lipovsky sees the texts with the eye of someone schooled in peoples and migrations more than as a textual scholar.
The time for the compilation of these stories would have been the United Monarchy. The motive would have been to give the people ruled over by David and Solomon a single genealogy and a common history.
The compilers were concerned to establish the birthright of Jacob over Esau and the birthright of Judah over Reuben, Levi and Simeon. They did this by relating disqualifying stories about Esau, Reuben, Levi and Simeon. Esau had sold his birthright and allowed Jacob to get Isaac’s blessing by deception. Reuben had slept with one of his father’s concubines. Levi and Simeon had slaughtered the people of Shechem and brought reproach upon Jacob. The Blessing of Jacob in Genesis 49 tries to pull all this together.
Lipovsky judges that these attempts did not work very well. They are easy to see through. Strangely enough, they prove the reliability of the compilers. If the compilers were free to just make stuff up, they could have done better. But they were constrained to use actual stories from the oral histories. So this was the best they could do.
This gives Lipovsky an unexpected conservatism about the historicity of the texts. For instance, the attack on Shechem must have been a historical incident.
The compilers were much more successful at pulling the family trees together.
They succeeded so well in intertwining the various pieces of narrative about Jacob and Israel, the forefathers of the southern and northern tribes, that all subsequent generations of the Jewish people considered Jacob and Israel to be a single forefather with a double name, Jacob-Israel.
Lipovsky says that the compromise the compilers found about the wives of Jacob was inspired. Rachel was actually a matriarch of the northern tribes. She was not buried with the southern patriarchs near Hebron. How, he asks, did the less favored wife, Leah, end up lying next to Jacob in the cave at Machpelah? The answer is that she really was Jacob’s wife. But she is presented as unloved, while Rachel is the second wife but most loved.
In this way the authors of the Pentateuch managed to preserve the primacy of the southern tribes as legal heirs while giving the northern tribes love and acknowledgement of their own special merits.