To which Bnei Yisrael is he sent?
Radak #1: Those who were in Bavel. They were exiled with Yehoyachin.
Radak #2: Perhaps it is also those who were in Yerushalayim; he will send written messages to them, for he was in Bavel 1 .
Radak: It seems that he was sent only to those in Bavel, for it says below (3:11) "v'Lech Ba El ha'Golah."
Who are ""Goyim Moredim?
Rashi: Two Shevatim 1 are called two Goyim.
Radak #1: They are Bnei Yisrael. They are one nation. They are called two, for they were divided in their evil deeds - these serve the idolatry of Bnei Amon, and these of Mo'av or another nation.
Radak #2: It says "Goyim" due to the division of Yisrael into Shevatim and families - "Amim Har Yikra'u 2 " (Devarim 33:19), "Acharecha Binyamin ba'Amamecha" (Shoftim 5:14].
Malbim: Yisrael do not normally rebel against Hashem. If they rebel, they are far from Teshuvah and it is hard to return them. Goyim normally rebel, but they are close to Teshuvah via a Navi - Ninveh proves this 3 ! Hashem gave two shortcomings. (a) They are Bnei Yisrael, and they will not accept a Navi's words. (b) In their deeds, they are [like] rebellious Goyim.
Why does it say "va'Avosam Pash'u"?
Malbim (3): This did not begin now. Their fathers Pash'u, and [the children] were Mored, which is worse than Pesha. This continued until today.