1)
Why does the Sefer mention Esav's descendants?
1.
'Rashi': it is for Yitzchak's honor. They are mentioned before Bnei Yisrael, to be concise and cast them aside.
2.
Radak: It is just like it mentioned Yishmael's descendants. It mentioned their kings before a king from Yisrael, for once there was a king from Yisrael, they were under Yisrael and did not have a king, until Malchei Yehudah sinned.
