Why are Tzedek and Mishpat the basis of His Kisei?
Radak: After saying that the world stands via Hashem's strength, it teaches that endurance of the world is via Hashem's Tzedek, Mishpat, Chesed and Emes. The verse repeats itself in different words, for Tzedek and Chesed are the same, and Mishpat and Emes are the same. "Mechon Kis'echa" is like a king who sits on his throne and sees the needs of his nation, and conducts them.
Malbim: Before, it explained that Hashem conducts His world with Chesed and Emes (or Emunah). Now it concludes that the Kisei on which He sits to conduct His lower creations is founded on Tzedek and Mishpat. Mishpat is acting according to the fixed law. Tzedek is going beyond the letter of the law. Then the judge does not consider the normal protocol, rather, the time, matter and litigants. Sometimes Hashem conducts His world according to Mishpat (fixed rules), and sometimes according to Tzedek. The root of Mishpat is Emes in the judge's Nefesh; the root of Tzedek is love of Chesed in his Nefesh's nature.
Why does it say "Yekademu Fanecha"?
Radak: This is the same as "Mechon Kis'echa", for Tzedek and Chesed that He does with the creations stands the world on its basis. This is Mishpat and Emes from him to fulfill His word - "Od Kol Yemei ha'Aretz Zera v'Katzir v'Kor v'Chom v'Kayitz va'Choref v'Yom va'Laylah Lo Yishbosu", "Zos Os ha'Bris
