Its not a good idea to attack a JW's faith, that will close their mind fast.
Very true. Before any discussion of Watchtower history you need to have left snippets of information and had casual talks to determine the persons level of interest and openness to even considering alternate viewpoints.
Every discussion with a JW must be done carefully, as it is very hard not to become confrontational, at which point the JW cult personality takes over and nothing gets through the defences.