1)
Why does it say that Shamayim va'Aretz will praise Him?
1.
Radak: This is a metaphor for increase of Simchah and praise in the world - "Yismechu ha'Shamayim v'Sagel ha'Aretz va'Yomeru va'Goyim Hashem Malach" (Divrei ha'Yamim I, 16:31).
2.
Malbim: The entire existence will praise for the salvation of Tziyon when David returns to his kingship.
