When I attended pioneer school almost thirty years ago, I learned a very effective technique in dealing with anyone who wanted to resist new ideas and thinking. Put the burden of proof on them. Jesus did that most effectively and it will work for you as well. How do you do this? Simply ask questions that require them to prove their assertions! For example, when it comes to 1914 you can ask, "Where is the number 2,520 found in the Bible? Why is it the basis for the calculation of the length of the Gentile Times if it is not in Holy Writ?" Witnesses may assert that the number is there. Then ask them to prove it. Where is it? If they hesitate, open a copy of their own Comprehensive Concordance to the NWT and challenge them to find it using this tool.
In this regard, if they try to use the book of Revelation's "time, times and half a time" being equal to 1,260 days and then say, "We just double those numbers to get 2,520" you counter with, "Ah, but John was specifically told to equate the two. Where was Daniel told that his seven times equaled 2,520?" Again, put the burden of proof right back on their shoulders where it belongs.
It is likewise with other basic Witness theology. "Show it to me in the Word of God and I'll believe it." Most Witnesses are so poorly educated in what the Bible itself actually says, they'll have a hard time answering your challenges. I did this with a Witness who once was my closest friend in the organization. The looks of fear and helplessness that crossed his face were most instructive.
When they assert that Matthew 24/25; Mark 13 and Luke 21 predict what we'd see in the last days of the worldwide system, get them to show you how that conclusion can be drawn when the passages clearly talk about the coming destruction of Jerusalem and its Temple and nothing else. If they want to talk about "God's organization", again, challenge them to find those words in the Bible. You can easily find the word "congregation", but "organization" is entirely and completely absent. The same is true with their "overlapping generation" nonsense.
If they want to reference the WTS publications, that's fine. But you need to remind them that the publications are not the Bible. Are they saying that the two are equivalent or that perhaps the publications have superceded God's Word? If they can't use the Bible alone to make their points, what other conclusion about the relative worth of the WTS publications versus the Bible would one draw? Using the questioning method should yield more than satisfactory results for you, and may get your Witness visitors to do some serious soul-searching.
Quendi