Spade, did you read The Gentile Times Reconsidered? If you are interested in reading it, send me a PM.
It presents a logical explanation to the 70 years, and that a 587BCE date for destruction of Jerusalem only conflicts with the Biblical 70 years if one insists on applying Jeremiah's prophecy to ONLY Jerusalem's destruction and lying desolate. But that's not the case. Jeremiah prohpecies that Judah, and the surrounding nations, would be in servitude to Babylon, a time period which started before Jerusalem's destruction:
(Jeremiah 25:8-11) 8 “Therefore this is what Jehovah of armies has said, ‘“For the reason that YOU did not obey my words, 9 here I am sending and I will take all the families of the north,” is the utterance of Jehovah, “even [sending] to Neb?u?chad?rez′zar the king of Babylon, my servant, and I will bring them against this land and against its inhabitants and against all these nations round about; and I will devote them to destruction and make them an object of astonishment and something to whistle at and places devastated to time indefinite. 10 And I will destroy out of them the sound of exultation and the sound of rejoicing, the voice of the bridegroom and the voice of the bride, the sound of the hand mill and the light of the lamp. 11 And all this land must become a devastated place, an object of astonishment, and these nations will have to serve the king of Babylon seventy years.”’
With the date of Jerusalem's destruction at 587 BCE, what Zechariah wrote in about 519 BCE:
(Zechariah 1:12) 12 So the angel of Jehovah answered and said: “O Jehovah of armies, how long will you yourself not show mercy to Jerusalem and to the cities of Judah, whom you have denounced these seventy years?”
would be written about 70 years later, versus a destruction of Jerusalem in 607BCE, which would have made the angel's words in Zechariah 90 years after the destruction of Jerusalem. The 1972 "Paradise Restored" book (chapter 8) has an extension discussion of this and acknowledges, and proposes, an explanation to the 20 year discrepancy. It does not however mention that the 20 years could be cleared up by moving the destruction of Jerusalem to 587BCE and understanding Jeremiah's 70 year prophecy as the nations servitude to Babylon. This explanation makes Biblical history and secular history agree.