insearchoftruth,
I think your best bet is to stick to the Isaiah book quote. The Watchter heavily relies upon the idea that 70 years is literal, yet in that book they say that a different 70 years was figurative without skipping a beat.
The less technical your argument is the better it will come off. Relying upon detailed chronology dating is not the way to go imo. You want something simple. I think the Isaiah book provides that. A few things to consider.
- Don't actually apply negative lables to your arguments. JWs typcially see everything as a battle between "good and evil". If they end up being wrong on a certain point, they automatically infer that you feel that they are on the side of evil (or something along those lines).\
- The main point about 1918/1919 is that the story promulgated by the Watchtower is a myth, something people within the organization slowly developed to explain and rationalize their own exsistance as a religion. This is something that is found in many faiths (modern ones such as the seventh day adventists and mormons are good comparisons, but traiditonal faiths like catholocism are good as well). Emphasis shoudn't be placed on assigning blame or casing negative accusations (e.g. The Watchtower are liars!), instead you should focus on how their beliefs are something that are common, i.e. "many religions create elaborate myths to validate their own exsistance, why are your bizzare stories backed up with dubious evidence any different?."
So using the Isaiah book quote as a starting point, you defend your belief that the 1918/1919 is something created by someone who had an interest to construct a myth to validate the Watchtowers exsistance (i.e. Rutherford). I would really focus on the individual (unknown of course) who thought up this idea. You can even ask rhetorical questions. Who was this guys? What were his motives? What did he (or his organization) seek to get by creating this doctrine or teaching?
You might also consider finding common ground. Get them to acknowledge first that other religions do actually create elaborte myths and stoires that validate the god given authority of the leaders of the group. One they agree that it has happened in other religions, you now turn it back to them. Why should the JWs be seen as different?