Larry,
I do not agree that the "70 years" ended with the return of the Jews.
The "70 years" relate to the period of servitude to Babylon by all of the nations that were named by Jeremiah. They all started their servitude together, at the same time.
I believe I have quite clearly shown that the 70 years was an unavoidable period of servitude whereas the destruction of Jerusalem was avoidable.
Even after Jeremiah instructed the exiles in Babylon to settle down for the 70 years had been decreed, Jeremiah pleaded with Zedekiah to surrender because by doing so the city would be saved.
Ezra says at 2 Chronicles 36 that the servitude ended at the moment Cyrus defeated the Babylonians. It makes full sense, since the servitude to Babylon ended when Babylon's power came to its end. As Daniel told Belshazzar, its days were numbered.
Cyrus was able to make his decree because he was now in power, the 70 years of servitude to Babylon had already come to its end.
This in turn indicates when the servitude to Babylon by all of the named nations began -- when Babylon became the regional superpower (or maybe in the religious sense, when God decided to use Babylon as his servant).
Doug