scholiar:
Technically speaking the original deportees had a longer exile than the next deportation but Jehovah had decreed that the Exile would be only seventy years not eighty because the exile was commensurate with servitude to Babylon and desolation of the land so could only commence with the Fall in 607 BCE. In fact we do not know how long that group lived in Babylon for nor do we have the demography of both groups in Babylon. Ezra of course does provide some demographics for the Returnees just prior to the Return in 537BCE. I hope this helps as you seem to be stuck on the figure '80'.
'scholar' likes to make broad sweeping claims about what the Bible supposedly says about the '70 years'. But I have already provided the specific scriptures (including Jeremiah 25:8-11 and 27:6-11) that clearly indicate that serving Babylon did not mean exile and that exile would be a consequence for nations that would not submit to Babylon. Of course, I have the advantage here, because the Bible never says there was a 70-year exile. It just doesn't. Never. Not once. There's nothing scriptural that 'scholar' can actually use to back up Watch Tower Society dogma.
However, the 'sting in the tail' is how the audience changes from verse 16 whereupon Jeremiah now turns his attention to the inhabitants of Jerusalem and gives similar warnings with consequences then in verse 20 it appears that he now addresses the entire nation as Exiles which of course would include both groups.
Once again, 'scholar' tries to claim the Bible says something it doesn't actually say. Verses 16-19 don't say anything about those still in Jerusalem being exiled to Babylon (which isn't the same as saying that wouldn't happen—it just isn't the context). The context of those verses about those still in Jerusalem is in response to the false claims by Hananiah (and the 'prophets in Babylon' at Jeremiah 29:15) about what would happen in Jerusalem (Jeremiah 28:2-4). It states that they would die or be dispersed among all the nations. It gives no indication at all that those exiled later would indicate a 'starting point' for the 70 years previously mentioned in verse 10. Verse 20 returns to addressing the people who are already exiled in Babylon, and says nothing about 'including both groups'.
'At Babylon' has equal status with 'For Babylon' sayin much the same thing with varying degree of emphasis. One rendering conveys location an dthe other conveys purpose. Both renderings are linguistically possible and both were realized historically and contextually.
It doesn't help your case that both renderings are 'linguistically possible' because the rendering you prefer is not contextually possible. 1) The alleged period from October 607 to October 537 in JW dogma includes travel time, so the Jews would be 'at Babylon' for less than 70 years. 2) Jeremiah 29:10-14 clearly indicates that the Jews would repent and then return only after the 70 years had already finished, so it would make no sense for attention to be turned to the Jews' return after that same period had already ended. The 70 years was a period during which all the nations were subject to Babylon, and not a period of Jewish exile. Jeremiah 29:10-14 quite straightforwardly indicates that 1) Babylon's 70 years would end (verse 10), 2) the Jews would repent (verse 13; compare Daniel 9:2-19), 3) the Jews would be allowed to return to Jerusalem (verse 14).
It's little wonder that 'scholar' doesn't regard arithmetic to be particularly important in regard to chronology.