1)
Why does it say "Shemosam b'Chatzreihem uv'Tirosam"?
1.
Malbim: Bnei Yishmael did not change their name, even after they increased and dwelled in open and walled cities.
2.
Ha'amek Davar: Initially they were minor officers, each was over a Chatzer. Later each became a major city. Even so, they kept their names, unlike Bnei Esav, who changed the names of their chiefs.
2)
Why does it say "Shneim Asar Nesi'im l'Umosam"?
1.
Ha'amek Davar: HaSh-m had blessed Avraham that Yishmael will father 12 Nesi'im (17:20). Even had He said 12 sons, we would know that each family will have a head who is like a Nasi over it. L'Umosam teaches that each head will be proper to be Nasi over the entire nation.
