1)
Why does it say "Lechu Shim'u"?
1.
Radak: Everyone says to his fellow Yir'ei Elokim, to make him zealous. This is like "Lechu v'Nakehu" (Yirmeyah 18:18).
2)
Which "Yir'ei Elokim" does he address?
1.
Rashi: Converts who will convert.
2.
Malbim: All of them.
3)
What is "Asher Asah l'Nafshi"?
1.
Malbim: Menasheh tells how Hashem accepted his Teshuvah, how His Hashgachah became known to him - "va'Yomer li'Yhudah La'avod Es Hashem" (Divrei ha'Yamim II, 33:16).