1)
Why does it say "u'Ven Ezrah"?
1.
Malbim: That was his name. We find "u'Ven Shimon", "u'Ven Zoches" (20). All of these were Bnei Yehalel'el.
2)
What is the meaning of "va'Tahar Es Miryam"?
1.
Radak: This is like va'Teled. Similarly, "Birchos Horai" is like Birchos Yoldai (my parents). Mered's wife bore Miryam to Mered (the coming verse proves so). Miryam was a son, like the others - we find names that apply to males and females, e.g. Aviyah Eshes Chetzron and Eifah Pilegesh Kalev.
2.
Malbim: It says va'Tahar to teach that she became pregnant with all three of them at once (they were triplets).
