Hellrider, I'm not sure how much I'm going to say here that you aleady know, but I'll say it for completeness.
By about 1900, it had been largely accepted in the scholarly world that Nebuchadnezzar had acceded the throne of Babylon in 605 B.C. Over the next few years, almost all scholars came to accept the modern figures for the regnal lengths of other Babylonian kings, thus establishing 539 B.C. as the last year of Neo-Babylonian kings and the date of Babylon's fall. Note that Smith's Bible Dictionary, as early as 1864, also accepted these dates (see http://corior.blogspot.com/2006/02/evolution-of-606-to-607-bce-in.html for details ), and that the Society's 1968 Watchtower article on chronology listed authorities as early as 1907 who accepted the 539 date.
The point is that it was establishing both Nebuchadnezzar's accession date and the lengths of the reigns of Neo-Babylonian kings that first established the 539 date. More specifically, Nebuchadnezzar's 37th year in 568/7 B.C. was astronomically established, and then his accession year in 605/4 B.C. was derived from it. In other words, an earlier date was established, and then the date 539 was found by working forward in time.
During this time, it became evident that the 539 date could also be established by working backward in time from an astronomically established date (althought the tablets from which the information was derived were extremely problematic in many areas, and not nearly as solid as the ones establishing the Neo-Babylonian kings), namely, the 7th year of Cambysses in 523/2 B.C. If that was Cambysses' 7th year, then his accession year, and the 9th of Cyrus, was 530/29 B.C., and Cyrus' accession year was 539/8. If 539/8 was the last year of Neo-Babylonian kings and Cyrus' accession year, then Babylon fell in 539 B.C. -- consistent with what had been first established by working forward in time.
The Watchtower Society is perfectly well aware of all this, and so the only reason for its wanting to retain the backward calculation of 539 and dump the forward calculation is that the latter clobbers its claims about the 607-1914 "Gentile times" doctrine. That's also why it wants to dump Ptolemy's Canon and refuses to discuss any of the solid evidence that the Neo-Babylonian king's reigns are correct in modern chronology.
Your arguments are exactly right, and scholar pretendus will never deal with them directly.