scholar:
Technically speaking the original deportees had a longer exile than the next deportation
'scholar' apparently imagines that it is technical that an event that starts earlier than another event is longer if both events end at the same time. No, 'scholar', that not technical. It's fundamentally obvious. No wonder he's scared of arithmetic.
but Jehovah had decreed that the Exile would be only seventy years not eighty
Wrong. The Bible never mentions 70 years of exile. Jeremiah 27:6-11 explicitly states that exile was a punishment for not 'serving Babylon'. The Bible does not explicitly provide the total length of the exile (it is given implicitly as about 60 years, from February 597BCE until October 538BCE, including travel time). However, the Bible does indicate the length of the period of paying off the 'sabbaths' at Leviticus 25:8 (to which 2 Chronicles 36:21 alludes) which is synonymous with the '7 weeks' of Daniel 9:25. According to the Bible, that period—49 years—ran from October 587BCE (2 Kings 25:26-28) until October 538BCE (Ezra 1:1; 3:1).
because the exile was commensurate with servitude to Babylon
Wrong. At the risk of repeating myself... Jeremiah 27:6-11 explicitly states that exile was a punishment for not 'serving Babylon'.
and desolation of the land so could only commence with the Fall in 607 BCE.
Wrong. The 'desolation of the land' was when it 'rested', which was the period of 49 years already indicated above. The beginning of that period aligns with 587BCE—the year in which Jerusalem was destroyed, however the exile was counted from 597BCE (specifically, the exile is enumerated from the year that began Nisan 598, because the exile began prior to Nisan of 597).
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.
Many Jews remained in Babylon, with no special distinction of either group (though most people from both initial groups would have been already dead). The only meaningful period of return relevant to the context of Jeremiah 29 is when the Jews were first allowed to return to Jerusalem.
I hope this helps as you seem to be stuck on the figure '80'.
AnnOMaly is stuck in reality. This obviously makes 'scholar' uncomfortable. (Of course, AnnOMaly doesn't believe the exile lasted for 80 years, because she recognises that the year assigned by the Watch Tower Society for the main deportation to Babylon is wrong.)