What does this Mizmor discuss?
Radak: All these Mizmorim until "l'David Mizmor" (101:1) discuss the days of Mashi'ach. This is connected to the end of the previous Mizmor. After all recognize that there is no injustice in Hashem, all will recognize that He is straight, and He is king over all.
Malbim: It is like the previous Mizmor. It discusses the two ways that Hashem conducts His world - fixed conduct based on nature, and miraculous Hashgachah according to the need and the time.
Why does it say "Malach" in past tense?
Rashi: In the future, people will say so in the future.
Why does it say "Gei'us Lavesh"?
Radak: People will no longer be haughty in front of Him like Nochri kings used to. Nebuchadnetzar said "E'eleh Al Bamasi Av [Adameh l'Elyon]" (Yeshayah 14:14); Pharaoh said "Li Ye'ori va'Ani Asisini" (Yechezkel 29:3); Melech Ashur said "b'Cho'ach Yadi Asisi uv'Chachmasi..." (Yeshayah 10:13); Melech Tzor said "Kel Ani..." (Yechezkel 28:2). All will recognize that only Hashem wears pride of kingship, and kingship is only to Him - "v'Haysah la'Shem ha'Meluchah" (Ovadyah 1:21), "v'Hayah Hashem l'Melech Al Kol ha'Aretz" (Zecharyah 14:9).
Radak (91:1, citing Shocher Tov): Moshe said this corresponding to Binyamin, who came in the shade of King of the world - "Chofef Alav Kol ha'Yom" (Devarim 33:12).
Malbim: Refer to 93:1:4:1 and the note there.
How does Hashem gird Himself with "Oz"?
Malbim: In Tanach, natural conduct is called Oz. It is constant strength that does not falter. Miraculous conduct is called Ga'avah; He is haughty over natural laws and overrides them, like I explained "Ta'oz Yadecha Tarum Yeminecha" (refer to 89:14:2:1). Even when Hashem clothes himself with Gei'us, to do wonders and tell the sun to stop, the sea to split, the rock to give water... He girds Himself with Oz; the natural laws on which the land is founded do not falter. It is clear from intellect that the latter shows His strength more than miracles, which are only temporary. There is more Gevurah in establishing that water always descends to the lowest place, that in piling up water at the time of Keri'as Yam Suf. Nature is a constant miracle - also it is due to Hashem's word! 1
Malbim: There is an advantage of a miracle over nature. At the time of the miracle, general nature persists. All parts of existence are tied to each other; movement of any one affects the rest, like a machine in which cylinders turn wheels, which move steps... If one will hold one part still, or push in the other direction, if the machine does not overpower him, it will cease to function. "Gei'us Lavesh... Oz His'azar" - Hashem does a miracle with one 'hand', and the other 'hand' continues natural conduct in all other respects, so the world does not falter - this is a great wonder!
How will He establish the land?
Rashi: Via reigning, he will make the land rejoice.
Radak: There will not be agitators of the land (tyrants); there will not be any more war. War is the opposite - "Por Hisporerah Eretz", "No'a Tanu'a Eretz ka'Shikor v'Hisnodedah ka'Melunah" (Yeshayah 24:19-20).
Malbim: Refer to 93:1:4:1*.
