Very interesting Christian Guy. That being said, though I believe many things you have said do indeed make sense, I strongly doubt on the accuracy and on actually shaking the faith of these witnesses. Here is why:
The JW are actually preaching, what they believe to be, the benefits of the Good News. That is, eternal life (in heaven and mostly on earth). These benefits, in their theology, are a result of Jesus’ sacrificial death and resurrection – which is the good news. Therefore, they indirectly preach the good news. The only thing however is that when you ask anyone that is not a JW what the JW preach about, they will tell you that they preach Armageddon brought about by their God Jehovah. Therefore, in reality, one could argue that the message they spread is not the good news.
As for the Mathew 24:14, some bible translation do render “Good news of the kingdom” as “Good news about the Kingdom”. That being said, I agree with your definition of the kingdom and I do believe that the Good news of the kingdom is about the hope for people.
As for ONE hope, well, I believe that hope is “forgiveness of our sins and therefore, a resurrection for the dead.” Where? I don’t care. Either way, I’ll be happy.