1)
Why does it say "Al Yisrael"?
1.
Malbim: The king who will eradicate Beis Yaravam will not be from Yehudah. Rather, he will be appointed after Yaravam over Yisrael.
2)
What is the meaning of "Zeh ha'Yom u'Meh Gam Atah"?
1.
Rashi citing Targum Yonasan: Those already born today, and those who will be born [will be cut off].
2.
Radak: This day that the new king will be appointed, and from now and onwards bigger afflictions will come on Yisrael.
