scholar:
What is false about your post is the claim that Daniel in this instance used a different calendrical system.
Wrong again, 'scholar'. Daniel's use of Nisan/accession dating is consistent throughout the book of Daniel, and it was the dating system used in Babylon where the story is set (and where Daniel was purportedly educated), the subsequent Persian period when JWs believe Daniel was written, and the Seleucid period when Daniel was actually written.
Jeremiah's use of Tishri/non-accession dating is also consistent for kings of Judah. There is insufficient information in Jeremiah to confirm whether he uses Nisan or Tishri dating for Babylonian kings (though likely Nisan consistent with 2 Kings), but he consistently uses non-accession dating (except for the Babylonian interpolation at Jeremiah 52:28-30, which is evident from the relative references to the 7th and 18th years).