Talk to the elder who said that he wont accept any research outside WT and ask him to prove the 607 date using only the watchtower publications.
He will end up smack dab at the insight publication that discusses the moabite stone, which is unfortunately outside information.
The process of astronomical dating is used to establish any and every historical date. The watchtower uses it too. ( or rather misuses).
The whole point is is that watchtower relies on outside sources. Point out to the elder that if he refuses outside sources he is really going against watchtower.
Good luck
wanna