What is "k'Shomerei Sadai"?
Rashi: It is like Shomerei Sadeh (guardians of a field). Radak - the Yud at the end of Sadai is in place of a Hei.
Radak: Sadai is like Sadim 1 or Sados. The Mem is omitted, like "v'Kara Lo Chalonai" (22:14) means Chalonim, "va'Chashufei Shes" (Yeshayah 20:4) is like va'Chashufim, and similar verses. Yonason translates like this. They are like people who guard fields, lest animals enter to graze. They must stand around the field, lest animals enter from any side. So the enemies will come around Yerushalayim. They camped in all Arei Yehudah around it. "Aleha" means around Yerushalayim, close to it, like "v'Alav Mateh Menasheh" (Bamidbar 2:20) and similar verses.
Malbim: It is like guardians of a field without people, for everyone fled, and broke through the wall on the way to Yericho, like it says in Perek 52 2 . They did not need to guard a city, rather, a field. This was not needed for other Arei Yehudah, only "Aleha" (for Yerushalayim).
Really, there is no word Sadim. The plural is Sados. Radak explains that Sadai is like a plural in which the suffix Mem is omitted, even though it is not Samuch. (PF)
Rashi (52:8): The king went via a cave to Arvos Yerecho. (The verse implies that all his legions fled with him, but later separated from him. - PF)
Why does it say "mi'Saviv"?
Malbim: They did not catch [Tzidkiyah and his legions] at the gate [of Yerushalayim], rather, around it - in Yericho.
Why does it say "Osi Marasah"?
Rashi: You angered Me. This is like "Mamrim Heyisem" (Devarim 9:7, 24) - you arouse anger.
Radak #1: You rebelled against My words via the Nevi'im. Hashem's word is like Himself. Also "Mamrim Heyisem Im Hashem" (Devarim 9:7, 24) is like Es Pi Hashem. The expression Meri is used with Es in front of the matter it connects to, e.g. "Merisem Es Pi" (Bamidbar 20:24), "v'Lo Maru Es Davaro" (Tehilim 105:28). Meri is changing or switching the matter or Mitzvah.
Radak #2: You rebelled against Me. It is as if it says "Bi". Meri is used with pronouns beginning with Beis, e.g. "Ki Maru Vach" (Tehilim 8:11), "va'Yamru Vi" (Yechezkel 20:8, 13, 21). Ba'al ha'Turim (from Mechilta 15:22) - one other verse says Marasah - "va'Yavo'u Marasah" (Shemos 15:23). Just like there, they rebelled against Hashem, also here.
Radak citing Targum Yonasan: You refused [to obey] My word.
Malbim: Bavel's army did not capture you so easily due to their strength or your weakness, rather, because you rebelled against Me. "Lu Chachmu Yaskilu Zos
