Why does it say "Tziyon b'Mishpat Tipadeh"?
Rashi: It will be redeemed from its sins via doing Mishpat.
Radak: Yisrael will be redeemed, and they will return to Tziyon.
Vilna Gaon: This is like it says "u'Va l'Tziyon Goel ul'Shavei Fesha b'Yakov" (59:20). Tziyon will be redeemed in Mishpat, for Hashem already struck it twice for its sins. Also refer to 1:27:3:1.
Malbim: The rebuke of Yerushalayim in verses 21-26 was primarily about matters Bein Adam l'Chavero, e.g. swindling and tilting judgment. Due to the great business there, they sinned more in these, so they will find redemption and success via Mishpat. The rebuke of Yehudah, before verse 21, was primarily for sins Bein Adam la'Makom, e.g. Korbanos, Teshuvah and Tefilah. People outside Yerushalayim sinned more in these. Their Tikun is in Tzedakah, i.e. to justify their deeds to their Creator.
Birkas Tziyon (ha'Gaon Ben Tziyon Aba Sha'ul), citing ha'Gaon R. Y. C. Zonenfeld: Its Gematriya is 1076, like "Talmud Yerushalmi" - this is its merit to be redeemed. Also refer to 1:27:3:2.
Who are "Shaveha"?
Rashi: Those in it who repent.
Radak: Those (Malbim - Shevet Yehudah) who were exiled from it.
Why does it say "v'Shaveha bi'Tzdakah"?
Vilna Gaon: They will be redeemed via Tzedakah, and not based on Mishpat (letter of the law). They asked, a Nefesh that sins
