Throughout this forum, there is a running debate over whether it is better to DA yourself, or simply "fade." There are a number of threads on tthe topic, and after several good points are made in favor of each, the consensus seems to be: Different strokes for different folks.
The same is true of the very fine, but conflicting, advice you've received here. Either Gary is right, and you should view your father as mentally ill and treat him accordingly, which gives you a sense of peace as well as the upper hand. Or, Vinny and Quandry have it right, and you should put forth a great deal of effort to become thoroughly conversant with Witness doctrine so you can lead them through the maze to enlightenment.
I'd say Gary's way is easier. And based your comments here, Schism, my guess is that you'd be way more comfortable taking his advice.