Why does it say "Mi Eloka..."?
Radak: Who can annul His word and what He commanded?!
Radak: This is unlike those who trust in their idols. Since I always overcome them, they should know that their molten images are false, and there is no Rock like ours.
Radak (Shmuel II, 22:32, based on Targum Yonasan there): At the time of Mashi'ach, all the nations will admit that there is none like Hashem.
Why does it switch from Eloka to Hashem?
Malbim: Shem Havayah alludes to the Creation; He was Mehaveh (brought into existence) everything found. Shem Eloka alludes to conduct and Hashgachah. Elokim, who conducts and supervises according to deeds, He Himself is Hashem who created the entire Creation and established the laws of nature. There is no power that creates and forms other than Elokeinu, who conducts and supervises. The conductor is the Creator, and the Creator is the conductor. Surely He arranged that there is no contradiction between the laws of nature and the laws of conduct. The former are drawn to conform to the latter.
Why does it say "u'Mi Tzur..."?
Radak: Who can turn back His decree?!
