Who says "Libi l'Mo'av"?
Rashi (Yeshayah 15:5): The Navi 1 . Nevi'ei Yisrael are unlike Nevi'ei Goyim. Bil'am wanted to uproot Yisrael for no reason. Nevi'ei Yisrael mourn over the punishment that comes to the nations!
Radak: Each person of Mo'av says so.
Malbim (in Yeshayah 15:5): This is a metaphor, as if the Navi's heart, which sees the end, calls to Mo'av's army going to war, and tells them not to fight, for they will not succeed. Rather, they should flee.
Rashi said so in Yeshayah, but surely the same applies here. (PF)
Why does it mention "Kir Cheres"?
Malbim: All the ill and wounded lied there.
What is the meaning of "Al Ken Yisras Asah Avadu"?
Rashi: Due to what pride did 1 , they perished.
Radak, Malbim: The wealth and multitude that [Mo'av] gathered was lost. Radak - the Tov in "Yisras" is in place of a Hei, like I wrote about Eglas (verse 34). It should say Asher Asah; the word Asher is omitted. Also "la'Am Aleha" (Yeshayah 42:5) is like Asher Aleha; "ha'Derech Yelchu Vah" (Shemos 18:20) is like Asher Yelchu Vah.
They refused to flee, or to request help. (PF)