1)
Why does it say "Barchu Hashem Kol Tzeva'av"? Verse 21 already commanded all His angels to bless Him!
1.
Radak citing Shocher Tov: Refer to 103:20:1:3.
2.
Malbim: This refers to the world of Galgalim (spheres that house the Heavenly bodies). They serve constantly, not only sporadically.
2)
Why does it say "Osei Retzono"?
1.
Malbim: They do not wait for a special word. The previous desire that Hashem wanted at the time of creation, this fixed their laws and established them forever - a law that will not be transgressed. This they do forever, without ceasing.
