What is "Mor'ah v'Nig'alah"?
Rashi: Until now discussed Ninveh. Now it returns to Yerushalayim. Woe that it will be plundered and stinking like excrement with its sin. Mor'ah is like "Mur'aso" (Vayikra 1:16), "v'Samtich k'Ro'i" (Nachum 3:6). Radak - this is filth. Also v'Nig'alah means like this, like "v'Chol Malbushai Eg'alti" (Yeshayah 63:3), "Nego'alu ba'Dam" (Eichah 4:14). The matter is repeated in different words.
Malbim: From when the Galus began via Sancheriv, Nevi'im began to prophesize about the Ge'ulah. From then, the Ge'ulah was possible, had they merited, like I explained in Yeshayah in Perakim 11 and 48. Had the Ge'ulah been in the days of Chizkiyah, Yoshiyah or Bayis Sheni, it would have been only for Yisrael. However, the Ge'ulah at the end of days will be for the entire world. Then all will recognize Malchus Hashem and His unity, and all will call in His name. After informing the fall of Ninveh and the nations that harmed Yisrael, and prophesizing that She'eris Yisrael will inherit Pelishtim, Amon and Mo'av, it teaches that earlier (in Yemei Chizkiyah, Yoshiyah or Bayis Sheni), Yerushalayim was awesome. Nations feared it, for Hashem was with it. It was Nig'alah from Melech Ashur. 1
Malbim: First was in the days of Chizkiyah. Hashem wanted to make Chizkiyah Mashi'ach and Sancheriv Gog and Magog, but a sin blocked it. In the days of Menasheh officers of Bavel came and incarcerated him. Melech Ashur came a second time, like it says in Nachum. In the days of Yoshiyah, Yirmeyahu thought that the time of Ge'ulah came, and he went to return the 10 tribes. Also when Ninveh was destroyed at the beginning of Yehoyakim's reign they did not fear Malchus Ashur. The Ge'ulah could have come, but they sinned, and Melech Bavel exiled them. Had all the exiles ascended [for Bayis Sheni], it would have been a permanent redemption. After Churban Bayis Sheni, it depends on the special designated time at the end of days. (Sanhedrin 98a said that if we merit, Achishenu (Hashem will hasten Mashi'ach); if not, b'Itah (he will come in the proper time). This implies that also after the Churban Sheni, he can come early! - PF)
Why is it called "ha'Ir ha'Yonah"?
Rashi: This is like "va'Yhi Efrayim k'Yonah Fosah Ein Lev" (Hoshe'a 7:11).
Radak: This is like "Lo Sonu" (Vayikra 25:17), "Cherev ha'Yonah" (Yirmeyah 46:16).
Radak citing Targum Yonasan: Woe that the city that will be redeemed quickly, it angered [Hashem] greatly.
Malbim: It is Yerushalayim, which was under the hand of Ashur and Bavel, and its flag had the form of a dove, and it was called so based on its kingship.