In my view, there needs to be a very clear decoupling of the destruction wrought on Jerusalem/Judah from the 70 years of servitude/Babylonian domination.
The destruction was the outcome of the people failing over many centuries to obey God, and it was a conditional prophecy, the outcome of which depended on the people's response.
However, the servitude to Babylon, experienced by several countries, was an unconditional prophecy. It would come to pass, no matter what.
At times the unconditional prophecy encountered the conditional prophecy ("Serve the king of Babylon, and you will live. Why should this city become a ruin?" Jer. 27:17 NIV).
The event that started the Seventy Years is identified in the direct context of the 70-year prophecy (Jer 25:11), with the answer given at Jer 25:9: “I will summon all the peoples of the north and my servant Nebuchadnezzar king of Babylon,” declares the LORD, “and I will bring them against this land and its inhabitants and against all the surrounding nations.”.
Since the “70 years” is the duration of Babylon’s supremacy, it is only natural that the period began with the moment the Lord summoned Babylon into that role.
The Bible writers were far more concerned at knowing when the people would return home, rather than trying to locate a moment in history when the Seventy Years commenced.
The end of the Seventy Years needs to be positively decoupled from the Jews' return. The end of the 70 Years is marked as the time Babylon stopped being the regional super power. But the return of the Jews depended on people calling on the Lord, not the other way around. Then, and only then, would the Lord respond. But they had to wait until the 70 Years had been completed. “For I know the plans I have for you,” declares the LORD , “plans to prosper you and not to harm you, plans to give you hope and a future. Then YOU WILL CALL UPON ME and COME AND PRAY TO ME , and I WILL LISTEN TO YOU . YOU WILL SEEK ME and FIND ME when YOU SEEK ME with all Your Heart. I WILL BE FOUND BY YOU ,” declares the LORD , “and will bring you back from captivity. I will gather you from all the nations and places where I have banished you,” declares the LORD , “and will bring you back to the place from which I carried you into exile.” Jer. 29:11-14. Hence the significance of Daniel's confessional prayer. However, since his prayer was not a national confession of sin or a national seeking of the LORD, although they were permitted to return, the initial 70 years of servitude was multiplied by a factor of 7. This time, however, instead of being a servant of another nation, they would be a servant of the Most High God.
Doug