What is "Shem Kevodo"?
Radak: This is His [four letter] name that is written but not pronounced. It is His name of honor. "Ba'Yom ha'Hu Yihyeh Hashem Echad u'Shemo Echad" (Zecharyah 14:9) - only on that day, it will be pronounced 1 .
Malbim: Shem Kevodo is aggrandized when Hashem conducts based on fixed [natural] order. All will recognize that He is the Creator and conductor. Shem Tif'eres is aggrandized when he does open miracles in the eyes of all - "La'asos Lecha Shem Tif'ares" (Yeshayah 63:14).
I.e. even commoners. Even beforehand, special people pronounced it, e.g. the Kohen Gadol on Yom Kipur, Moshe said it to kill the Egyptian... (PF)
How will His honor fill the land?
Radak: "Ki Mal'ah ha'Aretz De'ah Es Hashem" (Yeshayah 11:9).
Why does it say "Amen va'Amen"?
Radak: Affirmation and affirmation. This matter will truly endure.
Malbim: Amen corresponding to Shem Tif'eres that does wonders, and there will be lasting Emunah, and Amen corresponding to Shem ha'Kavod.
