Why does it discuss a Karchah (bald patch)?
Radak: Pharaoh struck [Azah] first, and afterwards Nebuchadnetzar made it Karchah - nothing remained. Malbim - Pharaoh uprooted some of its gates. Now the remaining gates will be uprooted.
What is the meaning of "Nidmasah"?
Rashi: It was destroyed.
Radak: It was cut off. This is like "Nidmah Melech Yisrael" (Hoshe'a 10:15).
What is "She'eris Imkam"?
Radak: Their great property and army. Emek refers to multitude, like "Hemiku Shichesu" (Hoshe'a 9:9), "Hamek She'alah" (Yeshayah 7:11).
Radak citing Targum Yonasan: The remnant of their strength. Radak - perhaps some of them dwelled in the valley.
Malbim: Ashkelon was already plundered. Now also [its] valley will be cut off.
What is the meaning of "Tisgodedi"?
Rashi: You will scratch [and wound] yourself amidst mourning and crying. Radak - this is like "Lo Sisgodedu" (Devarim 14:1). Malbim - it uses this expression for similarity to Karchah; the Torah forbids both of these amidst grief over a Mes (ibid).
Radak citing Targum Yonasan: Tis'hamemun (you will be confounded).