I hope no one minds an uneducated female sticking her nose in here, but looking into this topic was what initiated my leaving the witnesses.
The secular establishment of the 539 BCE date has nothing to do with the secular establishment of the 607 date. One can only arrive at the 607 date by applying the Watchtower's unique interpretation of scripture.
There are, however, thousands of pieces of evidence, legal documents, astronomical diaries, royal succession lists, which all fit together nicely placing the date of Jerusalem's destruction at 586/587 BCE. Literally thousands, making 586/587 an absolute date. Considered with the very reasonable explanations for the 70 years, the Bible is consistent with secular history.
The pieces of evidence you gave in your above response applied to the 539 date, which isn't in question. I haven't seen any evidence given that applies to the 607 date. Actually, if I remember correctly, there is much more evidence to establish the 586/587 date than there is to establish the 539 date.
The Society itself, in the 5/15/71 Watchtower, p. 316, "Testimony of the Nabonidus Chronicle", admits the Nabonidus Chronicle of itself doesn't provide the basis for establishing the year for the fall of Babylon. It depends on other evidence to complete the puzzle, which it does. But the WT doesn't seem to mind that there is no evidence whatsoever except their own opinion to establish the 607 date.
I'm not a scolar and just have my computer and library to teach me, but that's my $.02.
PennyCandy