1)
Why does it say "ha'Yoshvim b'Arei Yehudah"?
1.
Malbim: Only Yisraelim (from the 10 Shevatim) who lived amidst Yehudah [and Binyamin] accepted Yaravam for their king. The rest of Yisrael merely went to their tents; they did not have a king.
2)
Why does it say "va'Yimloch"?
1.
Radak: It means Malach. The Vov is superfluous, like "va'Yisa Avraham Es Einav" (Bereishis 22:4).
