I think the key is to remember you are trying to discuss opinions/convictions not facts.
When B says something like “C is a nazi”, A correctly asks why B believes C is a nazi, not why C is a literal nazi. So when you go down one level, A’s next question should be something like “why do you think these are nazi tactics?” and “why are nazi tactics bad?” It really requires both sides to be intellectually honest and curious about someone’s actual beliefs, otherwise the technique doesn’t work. I also think limiting yourself to just “why” isn’t always helpful. Sometimes you need to ask for clarification or the entire conversation becomes a farce.
Remember the goal is to learn something about the other persons views, not to set each other up with rhetorical questions.