First off WELCOME to this board, nice to have you here. I hope you stick around.
It's good that you've been playing along. If you calmly ask them innocent sounding questions and make them do their own research then you can help them by planting some questions in their mind. But it has to be done calmly, without confrontation otherwise the JW radar goes up.
That is what happened to a JW decades ago. Carl Olaf Jonsson. He had a bible student ask him to prove that 607bce & 1914 were the dates mentioned in the bible. He couldn't do it. Several years later he wrote the book 'Gentile Times Reconsidered' proving that WT was wrong. He was kicked out of WTBT$ for his research.
I don't know if you've come to the chapter yet in your study about God's name. They believe it to be Jehovah and have even taken it as part of their name. Here's some info for you. This comes from the Catholic answers website. http://www.catholic.com/quickquestions/is-gods-name-yahweh-or-jehovah
But you can ask them, "Well if JW's have gotten totally away from other religions, especially the Catholic Church, then why do you use the name Jehovah, when it was a Catholic Monk's INVENTION?"
They can try to explain it away all they want to, but they cannot answer that one!