What will Hashem not do?
Malbim: He chose the middle path. He will not distance to Shamayim and leave them Hefker in the hand of Melech Ashur, to eradicate them like Hashem initially decreed in His anger - "Lo E'eseh Charon Api."
To what will He not return?
Rashi: I will not retract from My good words - "Lo Me'astim v'Lo Ge'altim" (Vayikra 26:44). I will not retract to destroy Efrayim.
Radak: If I afflicted them for their sin, I will not return to totally destroy them.
Malbim: I will not return to Shomron, for then I would need to destroy it.
How does "Ki Kel Anochi v'Lo Ish" explain the reason?
Rashi: I fulfill My good word; I do not retract from good.
Radak #1: I am Erech Apayim - unlike a person, who cannot bear his anger, and does to his enemy according to his anger.
Radak #2: "Lo Ish Kel vi'Ychazev u'Ven Adam v'Yisnecham" (Bamidbar 23:19). Once I promised their Avos to do good to them and their seed, I will not retract from My promise, even though they sinned. I will afflict them, but I will not destroy them from being a nation.
Malbim: (a) It explains "Lo E'eseh Charon Api" - I am Kel, so I am gracious. (b) It explains "Lo Ashuv" - if I would return to the city, I would have to destroy it.
What is the connection of "b'Kirbecha Kodesh" to "v'Lo Avo b'Ir"?
Rashi #1: Because Kodesh is in you, Yerushalayim, I promised to make My Shechinah dwell in you, and not in another city. Radak - Rav Sadya Gaon and Targum Yonasan are like this.
Rashi #2: "B'Ir" is an expression of hatred, like "va'Yhi Arecha" (Shmuel I, 28:16. Because Kodesh is amidst you, I will not come with hatred.)
Radak citing Ibn Ezra: Kodesh is in you, even though I will not enter the city - "ha'Umnam Yeshev Elokim Es ha'Adam Al ha'Aretz" (Divrei ha'Yamim II, 6:18).
Radak citing Ta'anis 5a: Is "b'Kirbecha Kodesh" a reason why "v'Lo Avo b'Ir"? Rather, I will enter Yerushalayim above until I build Yerushalayim below. There is Yerushalayim above - "Yerushalayim ha'Benuyah k'Ir she'Chuberah Lah Yachdav" (Tehilim 122:3).
Malbim: This is the middle path that Hashem chose - He will not distance to Shamayim. Rather, He will descend to be among them, to guard them. However, He will not enter the city; He will stand outside, and take them outside to go to Galus. The verse teaches that I will leave Kodesh in you and not distance, but I will not enter the city.