Why does it now discuss Mashi'ach?
Radak: The verses discuss the final redemption.
Malbim: The verse discussed the returning of the nation and the returning of leaders, which were fulfilled in Bayis Sheni. Corresponding to the latter, it says that there will come a time when these promises will be fulfilled with the ultimate happiness and success.
Why does it say "Tzemach Tzadik"?
Radak: It is Mashi'ach. He is called Tzemach, for he will go out in the world like Tzemach ha'Sadeh (vegetation) sprouts - it increases greatly. Just like he will be a Tzadik, with him most of the world will be Tzadikim - "Yifrach b'Yamav Tzadik" (Tehilim 72:7), "v'Hochi'ach l'Amim Rabim" (Yeshayah 2:4).
Malbim: He will not be called a shepherd, rather, a staff will sprout from its source - "v'Yatza Choter mi'Geza Yishai" (Yeshayah 11:1).
Why does it say "u'Malach Melech"?
Radak: It is doubled to strengthen the action. When it mentions the action (reigning), it mentions the noun (king) This is like "Lema'an Botze'a Batza" (Yechezkel 22:27), "me'Hamtir Alav Matar" (Yeshayah 5:6), "Kol Bogdei Vagad" (above, 12:1). Here it says "Melech" to strengthen the kingship. He will rule in all the land, not only in Yisrael - "u'Mashlo mi'Yam Ad Yam umi'Nahar Ad Afsei Aretz" (Zecharyah 9:10).
Malbim: He will not be called a shepherd, rather, a great king.
What is the meaning of "Hiskil"?
Rashi citing Targum Yonasan: He will succeed. Radak - the same applies to "va'Yhi David b'Chol Derachav Maskil" (Shmuel I, 18:14) and other verses (Malbim - "Yaskil Avdi Yarum v'Nisa v'Gavah Me'od" (Yeshayah 52:13).
Will he do Mishpat and Tzedakah only in Eretz Yisrael?
Malbim: It will be in the entire world - "Lo Yisa Goy El Goy Cherev" (Yeshayah 2:4).
This is repeated below (33:15), but there it says "Atzmi'ach l'David Tzemach Tzedakah"!
Sanhedrin 98a: There are two ways that redemption can come - "b'Itah Achishenah" (Yeshayah 60:22). (a) If they merit, Achishenah (I will bring it early). (b) If they do not merit, b'Itah (it will be in its time). It says "va'Aru Im Ananei Shmaya..." (Daniel 7:13; Mashi'ach will come quickly", and it says "Oni v'Rochev Al Chamor" (Zecharyah 9:9 - slowly)! If we merit, he will come quickly. If not, he will come slowly. Malbim - here it discusses if we merit, so it says Tzemach Tzadik. There it discusses if we do not merit, so it says Tzemach Tzedakah. The supreme Tzedakah will not look at the righteous acts and merit. Rather, it will be due to Chesed and arousal from above. This is why it says there Atzmi'ach, for it is a new sprouting. Here it says va'Hakimosi, for it already sprouted from Yisrael's deeds; Hashem needs only to erect it from its falling.
This is repeated below (33:15), but there it adds "uva'Es ha'Hi", and it omits "u'Malach Melech" and "Hiskil"!
Malbim: There it discusses if we do not merit to bring redemption early (refer to 23:5:6:1). We will not need to wait for the designated Es (time). And Mashi'ach will not rule immediately; initially, he will be "Oni v'Rochev Al Chamor" (Zecharyah 9:9; Sanhedrin 98a). Here it discusses if we merit; he will rule immediately.
