"So, it's not just 607 that the WTS is at odds with modern historians but the whole of the kings of Israel chronology. How do they get away with this? What sources do they use?"
Say what? This is not news. This is how they have calculated it since Nelson Barbour came up with the 70 years from Restoration of jerusalem to destruction. 537-607 for the destruction of Jerusalem and then worked back. There is a 20 difference between that and the 586ers. Russell took that then Rutherford and everyone since that time.