It would appear ... interpretation is required ... it is not clearly stated ... if Ezra used the Spring calender ... if Ezra used autumn calender ... we cannot be certain
A bunch of speculation. This is the basis on which you consider the 70 years to have begun in 607. It's laughable. And you expect other people to accept your nutty religion based on that?!
And your interpretation of the 70 years is still entirely wrong anyway, because it was a period during which "all these nations" would "serve the king of Babylon", and not a period of Jewish exile. Based on the direct statement at Jeremiah 25:12, there is simply no valid way of shifting the end of the 70 years beyond the fall of the Babylonian empire in 539 BCE.
there is a firm view that 538 BCE is highly unlikely
Whose "firm view" are you citing?
Despite your claims, 538 BCE fits perfectly.