1)
Why does it say "v'Ir Lo Simalet"?
1.
Radak: No city will escape. The plunderer will come to every city.
2)
What is the meaning of "Asher Amar Hashem"?
1.
Radak: This is like Ka'asher Amar Hashem. Also "Tochal Matzos Asher Tzivisicha" (Shemos 34:18) is like Ka'asher.
2.
Malbim: They will be destroyed because Hashem said so.