Conflicted, I'm in a similar situation with my parents, except for the fact that they already know where I stand with regards to the WTS. They know I'm not just 'doubting.' For the first few years after I quit, I wouldn't have any debate with them. Then one day when I finally did, I found I couldn't do it and keep my emotions in check. Anyway, we just don't talk about Watchtower related things since then.
But, maybe there is dissonance in their minds. Only thing is they're trying their hardest to suppress it with meetings, service, and personal study. But none of that can change the fact that they are getting old in this "system," something they never thought possible. I told them that world conditions could get bad in the near future, not because of some imaginary evil being, but simply because of real environmental and human limits. I know they're looking for bad news as proof that these are the "last days." I said there will be no Jehovah, Jesus, or Paradise Earth to fix things if civilization as we know it suffers a collapse. (And collapse in human societies is not uncommon, even though we could know a scale of collapse unheard of in human history.)
So, if you must debate your parents, try to stick with the simplest arguments possible. For example, why do the scriptures say that Jesus is mediator for all mankind, but the WTS says he is not? Why wouldn't Witnesses tell people about that in service? How can it be called "the good news" when Witnesses are taught that billions of people are going to die very soon? If God really loves mankind, why can't he figure out a way to bless mankind without committing mass murder? Isn't it a bit unfair to place this burden (preaching salvation or damnation) on a few million people? Or is the preaching work really just a way for a publishing company to get recruits and make money on real estate and donations? (I mean, really, the whole WTS infrastructure is very wasteful of time and resources considering how tediously repetitive their "message" is.)
