1)
Why does it say "Ki Kel Gadol Hashem"?
1.
Radak: This is said for Yemos ha'Mashi'ach, when no one will deny; nursing, some deny His ability or Hashgachah.
2.
Malbim: For Goyim, He is Kel Gadol and Melech Gadol. Kel refers to ability; Melech refers to conducting and authority.
2)
What is "[Melech Gadol] Al Kol Elohim"?
1.
Radak: These are the Heavenly legions, which are Elohim and judges in the land. Then they will recognize that He is king over them, and they will act only as Sheluchim for Him.
2.
Malbim: They are powers of nature. Since He created all of them, He is king over them - He conducts the world via His Hashgachah.
