Who says "Achen..."?
Radak #1: They (the freed captives from Kush and Egypt).
Radak #2: The Navi says that the Nochrim will admit this.
Why is Kel called "Mistater"?
Rashi citing Mechilta: So they will say to Hashem. You made us understand that You hide from showing Your [ability to] triumph, as if You lack ability, in order to collect the debts of Your nation.
Radak, from Targum Yonasan: This is like Nistar (hidden). In truth, You are Kel, who makes Your Shechinah dwell in a high mighty place.
Radak citing Ibn Ezra: This is Hispa'el conjugation. You are hidden from a nation that You do not desire.
Malbim: You seem like one who hides from his friend, lest he see him. So You hide due to sins of Yisrael, temporarily, to the point that they do not see Your Hashgachah over them. Really, You see them, and supervise over them from the holes [in the wall].
Why is He called "Elokei Yisrael Moshi'a"?
Rashi: When Your mercy is aroused, You are Elokei Yisrael, who saves.
Radak: Your strength is seen them. You save them at the time of their affliction.
Malbim: Even when You hide due to their sins, You are Elokei Yisrael, who saves.
