1)
Why does it say "Melech Kol ha'Aretz Elokim"?
1.
Malbim: Also his simple conduct via Shem Elokim, he is now king over the entire land, and all serve him.
2)
Who is "Maskil"?
1.
Radak: It is everyone who gets wise - both Yisrael and other nations. Even though all nations are commanded to shake hands and sing to Hashem (verses 2, 7), even those that are not wise, to compose a song is only for the wise.
2.
Radak citing his father: Sing a song that will make people understand that Hashem alone is King.
