Why does it say "Al Har Gavoha Ali..."?
Rashi: This promise of the tidings is from the mouth of the one who lives forever.
Radak: This is a metaphor. One who wants to make his voice heard, he ascends a tall place.
What is "Mevaseres Tziyon"?
Rashi: A Navi who will inform Tziyon.
Radak, Malbim: It is Tziyon, the herald. The Nevu'ah depicts that it informs the other cities.
Here it says "Mevaseres Tziyon" (feminine). Why does it say below (52:7) "Raglei Mevaser" (masculine)?
Rashi: If they merit, [salvation] will be fast, like a male; if they do not merit, it will be weak like a female, and delay until the deadline.
Why does it say "Mevaseres Tziyon" and "Mevaseres Yerushalayim"?
Radak: They are the same. Yerushalayim is the primary place of Eretz Yisrael, and also Tziyon. They are the same city.
Malbim: Tziyon is the place of the king, the Sanhedrin and the Mikdash. The masses dwelled in Yerushalayim. Tziyon will herald that He returned His Shechinah to Tziyon, and the kingship and Kehunah 1 - "Al Har Gavoha Ali Lach". Yerushalayim will herald the gathering of exiles. Therefore, it says "raise your voice", to gather those who were cast off to the four corners of Shamayim.
Even now, Isurei Kehunah apply, and Kohanim give Birkas Kohanim, and we honor them. However, the primary law of Kehunah is Avodas ha'Mikdash. Also, nowadays only a small number of people are Vadai Kohanim. (PF)
Why does it say "Al Tira'i"?
Radak: Do not fear to herald, and tell Arei Yehudah "behold, your G-d!" He will surely come - i.e. His honor will return to Yerushalayim. It mentions Yehudah due to Yerushalayim; also, kingship is of Yehudah.
Malbim (8-9): Do not fear lest the matter not come. One can trust in something only if it is strong, and persists. Both of these apply to Hashem; there is no fear lest His word not be fulfilled!
Why does it say "Hine Elokeichem"?
Malbim: It is as if you show His place with the finger, i.e. it already occurred.
