1)
Who says "Hashem Hoshi'ah..."
1.
Radak: They would say so in war - "b'Shem Hashem Elokeinu Nazkir" (verse 9).
2.
Malbim: David says it. Refer to 20:10:2:2.
2)
Who is "ha'Melech"?
1.
Radak: It is Hashem, King of the world, who has the ability the save.
2.
Malbim: It is David. When we call in prayer for the sake of ha'Melech David, he will answer us and say "Hashem Hoshi'ah." I.e. I do not trust in army or power, only in Hashem's salvation, for He alone saves.
