The easiest way to keep things straight with people like JWs, Mormons, and others that claim that Christianity apostatized in theology is to reason on issues that they can not defend. From a scriptural debate we could sit and argue forever whether or not Jesus is equal to the Father or lower. We could argue forever whether or not the wicked suffer eternally or are annihilated. JWs could use scripture to make almost any claim on subjects that are not explicitly described in full detail. We know that Christ himself claims that He purposely spoke in parables so that not everyone would know the fullness of the Gospel unless they were part of the Church. It is actually a very wise thing He has done, in that we can recognize false teachings easier. If evil men knew everything about true Christian doctrine, they could be a lot more dangerous.
I would not go much beyond the fact that their own reliance on “The Bible” means that all things written in it are true. This gets tough for them when it is very clear that Jesus would be with His Church until the end of time, and that Hell would not prevail against it. This means that it has never completely gone away as the cults claim. It means that the understanding of who Christ is could not have changed in the late 1800s. They believe that anyone that is not a Jehovah’s Witness is a false Christian, needing to be truly baptized into the JW religion. So they do in fact say that there are no JWs before them. The span from the 1st century to the late 1800s have no true Christians to them by their own logic. The other is Christ’s warning that false prophets would claim that Christ has come again already at a set time. JWs claim that Jesus returned in 1914. They also predict when He will act to destroy the wicked to be in a literal generation from 1914, they have had to modify their definition of generation multiple times now since 100 years has passed from that day.
If they cannot explain these simple contradictions, why would anything else they say carry any weight? I like to use the idea of someone that picks up a board game with all of it’s pieces but without any instructions. They could play a game for sure, they might even get close to the game developer’s intent, but at the end of the day, all they have done is created their own made up game using someone else’s material. For fantasy and fun this is okay, but when our lives depend on this, it is very dangerous. Their “game” they have created could be an interesting, somewhat reasonable game, but if it is not what the developer intended, then it is the wrong game.