Thinking about this a little bit more, I feel like part of the problem is that the word "truth" is so loaded by the cult. "Truth" to a JW means "JWism" so arguing that JWism was not true at some point causes them to do some mental gymnastics to defend JWism because their brain hears this sentence as "the truth isn't true" and it doesn't make sense to them.
Perhaps a better way to approach this is to use a synonym of truth (fact/accurate/etc) that doesn't fire off the same rigged mental pathways the way the word "truth" does. Unfortuantely even that's somewhat of a minefield - words like accurate can be quickly associated to the oft used phrase "accurate knowledge of truth" which leads you right back to where you don't want to go.