1)
Who will stumble, and on what?
1.
Rashi: On those rocks, these (Pekach and Shevna) and their supporters will fall into the hands of their enemies. Hoshe'a ben Elah killed Pekach, and 10 tribes fell into Sancheriv's hand. Shevna left Yerushalayim when Sancheriv left it to go to [fight] Tirhakah Melech Kush. He inundated Shevna and his supporters, and left.
2.
Radak: On the Even Negef and Tzur Michshol mentioned, many - the rebels - will stumble. They thought to be saved via their rebellion; they will fall into the trap and be caught.
2)
Why does it add "v'Nokeshu v'Nilkadu"?
1.
Malbim: This corresponds to l'Fach ul'Mokesh.
