What is the significance of the five sons of Midyan - 'Eifah Eifer, Chanoch, Avida and Elda'ah' listed here?
Rashi (Bamidbar 25:16): They became the five tribes of Midyan mentioned there. The king of the third tribe was Tzur (the father of Kozbi).
Why does it say "Kol Elu Bnei Keturah"?
Malbim: This teaches that their lineage does not follow Avraham.