Who sent?
Rashi, Targum Yonasan: Shar'etzer v'Regem Melech va'Anashav. Radak - it says va'Yishlach (singular) due to the Klal. The same applies to "ha'Evkeh... Ka'asher Asisi" (3).
Radak #1: The verse does not say who sent.
Radak #2: Beis Kel; it is a person's name. He sent Shar'etzer, Regem Melech and his men. Regem Melech is a person's name.
When did they send?
Radak: This Nevu'ah was after they sent. The same applies to "Atah Katzafta v'Necheta" (Yeshayah 64:4; after we sinned, You became angry), "va'Yered me'Asos ha'Chatas" (Vayikra 9:22).
What is the meaning of "va'Yishlach Beis Kel"?
Who are "Shar'etzer v'Regem Melech va'Anashav"?
Rashi: They were Tzadikim. They sent from Bavel to their relatives in Beis Kel to seek Hashem's presence in Yerushalayim on behalf of them, and ask when they should cry in Av, since they returned to build the Beis ha'Mikdash.
Malbim: They were famous people in Bavel.
