I can't answer your question directly, but I can provide a little more context.

First, the classic, Wellhausen form of the Documentary Hypothesis has been critiqued over the past couple of decades. What this means is that contemporary scholars have moved away from seeing the four sources as discrete, independent works that were successively redacted into the Pentateuch. Instead, scholars tend to see the Deuteronomist and the Priestly Source as the organizing forces for their respective parts - the Deuteronomist writing the historical books of the Former Prophets, and P the Tetrateuch. These sources created the overall narrative, and then drew in information from the bodies of myth and legend that make up J and E, some independent works (like lyric poetry and a subdivision of Leviticus called the Holiness Code), and presumably some existing histories or court records.

So it's likely that P and D weren't just rewriting and editing sacred texts with a heavy hand. They were probably the ones who created the story.

The other thing is that D and P are apparently products of religious-political reform movements. This wasn't just the organic development of their religion. It makes sense that, for example, people wanting to centralize Yahweh worship in Jerusalem or formalize monotheism would try to cast and recast their stories to provide a basis for their programs. Maybe there's another part of this story that I'm not aware of, but I don't think there's any equivalent power dynamic for Christians speaking through the texts of classical Greece and Rome.

That depends on the period, classic author, and politics. In general, through, scribes included certain classic texts because they were useful in support of Christian doctrines, especially in the formation of early theology. For example, Aristotle gave the young pupil a rigorous training in logic which helped him apply certain teachings of Plato or Plotinus in shaping and defending the new faith's theological positions. The early church had to fight countless debates against learned pagan men and it usually came down to who essentially knew and could apply their Aristotle better, at least in terms of logic and rhetoric. Basically classic writers were held in such high esteem for the service they provided to the new faith that it would it have been in the scribe's interest to faithfully preserve the original words .