The fact that Jeremiah was set apart as a "prophet to the nations" (1:5) does not support your claim that "the Gentiles" = "the northern tribes." Indeed, there is a lengthy section of Gentile oracles in chapters 46-51, directed against nations who clearly were not the northern tribes.
Also, you have not addressed all the passages in the NT wherein "Gentiles" cannot possibly mean the dispersed northern tribes of Israel.
"Prophet to the Nations" was a title, not his mission or purpose. Consider this word of the Lord; "I am a father to Israel, and Ephraim is my first born." Consider that "Israel" in the OT refers more often to Ephraim (the 10 tribes of Jacob) and not to the Jews.
If "Gentiles" refers to anyone "not a Jew", such as heathens or unsaved pagans.... why is the word capitalized as if it refers to a specific group?