“there is not a single extra word contained within the entire Gemorah!”
This one is difficult.
First, there are in fact entire passages that are repeated word for word. AFAIK, nobody has ever drawn halachic or hashkafic conclusions from the fact that the gemara sometimes repeates itself.
Second, we aren’t even sure of the correct text of the gemara. Some rishonim are known to have had gemara texts that vary from our standard contemporay text.