Awakened07, you raise some interesting questions about what the JW responses could be that I'd like to comment on. You said:
Obviously, I wouldn't have been able to show it using only the Bible, so in that sense I would be stumped. You could quote literature that says they only believe the Bible, as you say. But back then, I would probably at least have justified it to myself; that based on what I already believed and had learned, the FDS are the modern day 'chosen people', and as such are those who should interpret the Bible. I would probably have thrown in the passages that say that "it is not in man who walks to direct his steps." (Jer. 10:23) and “no prophecy of Scripture comes from someone's own interpretation.” (2Pe. 1:20, 21) After some mental gymnastics, I would conclude that the GB/FDS are needed to understand scripture, and that if they say it is so, it is so.
I think that there needs to be a division drawn between two differant things that are often mixed togeather.
The response you gave I believe would be pretty typicial. After being asked such a question the Witness would have no choice but to begin to talk about how we need somebody to teach us. But I believe that the question I raised actually has little to do with whether or not it is proper for people to always have teachers.
The key I believe is not religoius authority, rather it is Watchtower history as a necessary component to understanding the Bible. Once it is established that this question cannot be answered without refering to Watchtower History, a new line is drawn. No longer can the Witness claim to believe anything but the Bible. There is now a secondary component needed to interpret scripture fully. That compnent is Watchtower history.
A good follow up question would be "is it possible for somebody living today to understand the Bible apart from Watchtower history?" I believe this is a key question. While on the one hand the Watchtower continues to promote the myth that they believe nothing but the bible, they also continue to promote the belief that a full understanding of the Bible can only be reached by having access to their religions history.
If I was talking to a JW I would first ask them if they believe nothing but the Bible. They of course would say yes. They may even bring this point up by themselves if you ask them "what do you believe?". I then would ask them my original question as stated above. I then would try to see if they would accept the proposition that one can only understand the Bible fully if they have Watchtower history to refer to. I then would ask why they falsely stated that they use nothing but the Bible when it is very that they have added an extra component (Watchtower history).