Vulcan, I see what you mean, but how about rather than showing that 607 is wrong, just show them that 607 is not relevant by asking them to answer a simple question:
What is the scriptural connection between the dream of Nebuchadnezzar and Jerusalem's destruction?
Answer: there isn't one. It is invented. They will try to show you the next scripture in the series but then you ask: Why is that scripture connected with Nebuchadnezzar's dream?
Answer: There is NO connection. It is invented.
Then tell them you will assume a connection exists (even though they can't find one) and ask them to keep explaining the doctrine. When they pull out the next unrelated verse you ask again: Why is that scripture connected to the previous two?
Eventually the cognitive dissonance will be too much to bear. THEN you tell them that EVERY SCHOLAR says 587/86 is the date, not 607, so EVEN IF the made up connections were true (which he's already now doubting) the WT starts at the wrong date.