Beniguman,
Again your whole premise (that whenever someone says or writes that God is speaking through them there is a 50/50 chance that he is) is false. The chances of God (even assuming he exists) actually dictating anything to any particular person are extremely small.
When someone says God dictated something to them they are saying that something that has very little chance of occurring has in fact happened to them. That is why when someone on a street corner tells us that he has had a conversation with God we write him off as insane by default.
If you want us to believe that a book was dictated by God and is not just another construct of humans (like the Koran or Book of Mormon or any of the other multitude of religious writings that claim to be dictated by angels or gods) you need to bring evidence that that particular book is different. An example would be something written in it that could not have possibly have been known by humans at that time (like the speed of light for example) would be evidence that human beings didn’t write it. The prophecies you keep mentioning are vague enough that there are endless discussions in the Gemara simply trying to figure out how, when and what those prophecies are even reffering to. The fact that people believed something was divine for many years doesn’t increase the chances that it is either. It a totally irrelevant point.