Isn't the time from the fall of Jerusalem to the completion of the second temple in the 6th year of Darius (Ezra 6:15; Zech. 7:5) 70 years (586 BCE to 516 BCE)?
Near as darn it. The temple was completed right at the end of Darius' 6th year - March (Adar), 515 BCE.
Doesn't this solve the Daniel 9:2 "desolation of Jerusalem" = 70 years issue?
Not quite. Daniel was checking Jeremiah's prophecy and his prophecy was all about the nations serving Babylon for 70 years. Neb. first invaded the land in 605 and 604, and over the next 66 or 65 years (to 539), with successive campaigns (n.b. Daniel said 'devastations' or 'desolations'), devastated and depopulated Jerusalem and its surrounding country.
Oh and the 607 or 606 date that was banded about by adventists and similar number-crunchers - it was understood to correspond to about the BEGINNING of Nebuchadnezzar's reign. Russell dug his heals in and said (paraphrasing), 'No, there's an 18 year discrepancy because the Bible talks of a 70 year desolation without an inhabitant, so counting from 536 (then understood to be Cyrus' 1st regnal year over Babylon), 606 gives us the year of Jerusalem's destruction, i.e. Nebuchadnezzar's 18th regnal year. It all fits, see?'