This thread should catch the attention of posters "scholar" and "thirdwitness" if they are still monitoring this forum. Back in this forum’s heyday, this thread would have turned into a 20 page battle between "scholar" and "AlanF" - which were actually a bit entertaining at times.
Personally, I believe that the 607 / 587 issue is a bit of a diversionary topic which causes one to spend enormous amounts of time battling over scholarly / academic minutia. I believe that one can loose sight of the big picture.
I think it is best to focus one’s attention on the big picture issues such as the Watchtower’s history of false predictions, their use of mind control, the presence of legalism and traditions systemic in the organization, etc. These are issues that the 607 apologists cannot easily grapple with; as most of them choose not even to discuss such issues.
Another major issue that JWs have enormous difficulty debating is the concept of Biblical inerrancy. They are not able to adequately discuss all of the overwhelming scientific evidence that humans have been on earth much longer than 6,000 years, and that a global flood (as described in Genesis) is utterly ridiculous.
The sole purpose to defend the 607 date is the uphold the teaching of the 1914 parousia. The sole reason to uphold that teaching is to uphold the teaching of Christ’s selection of the Watchtower as his earthly proxy. Much time and effort can be wasted on this issue due to the lack of any "smoking gun" evidence for either the 607 date or the 587 date. It’s a debate that could be waged by both sides ad infinitum.
In my opinion, it is best to focus one’s attention on the big picture issues that JWs cannot provide valid answers to.