It was natural to combine the text of PE with strands of JE, giving the background of the text and containing Bereshit 15:13 and Bereshit 25:26,
as explained here. These strands of JE became part of PE, and later of the P scroll. When the redactor combined P with JE, these strands were readily recognized. This can explain the apparent gap in P regarding the birth of Ya'akov and Esav. It was in the P scroll, but was a strand of JE. The next post is a possible combination of PE with strands of JE. The bold Pasuk would have been added by PE to JE, an introduction to the text that describes the life of Yitzchak. P has two such introductions:
25:19: "Avraham begot Yitzchak. And Yitzchak was ..."
25:13: "And it was after Avraham death and G-d blessed Yitzchak, his son."
It makes sense that one of the two is from PE.
