1)
Why does it say "va'Shem l'Olam Yeshev"?
1.
Rashi (from Shocher Tov): His name is complete and His throne is complete, like it says "Kis'o". Before [Amalek] is destroyed, it says "Ki Yad Al Kes Kah" (Shemos 17:16) - His throne is lacking, and His name is divided (incomplete).
2.
Radak: These and these will perish, and Hashem will always sit and judge these and these. "Yeshev" is endurance, like "Hashem la'Mabul Yashav" (29:10).
3.
Malbim: After He prepared His throne of Mishpat, He will not rise from it. He will always sit and judge all His creations - even after their memory was forgotten.
2)
Why does it say "Konen la'Mishpat Kis'o"?
1.
Radak: This is like "Hemah Yovedu v'Atah Sa'amod" (102:27).
