"If the whole thing was made up, ....don't you think somebody would have popped the question: "Why didn't I ever hear about all this stuff before? My grandparents never mentioned any of this?""
Do you know how legends are formed? There may or may not be a nugget of truth at the very beginning, but each succeeding generation of an oral tradition adds details, embellishes events, aggrandizes the reality. Have you ever played the game "Broken Telephone"?
The first time the story of Sinai appears was not in its written form, but in the minds and mouths of thousands of ancient Hebrews. Their grandparents did mention something like it as did theirgrandparents before them. It was this fluid oral mythology which was eventually written into the Torah which we know today.
George Washington's existence has many independent sources. Hell, we even have his teeth. I don't count "eyewitnesses" who don't leave arecord of what they saw. The Torah has a record of a mythology which states there were witnesses. Hardly the same thing.
"That, my friend, is the basis of all historical validity. Numbers. The more witnesses, the truer the story."
If I write a book and claim that 6 billion witnesses saw me fly, does that make it truer than the Torah?
"Who told you that? Please provide evidence that the first time the story of Sinai appears was not in its written form, but in the minds and mouths of thousands of ancient Hebrews."
That's how mythology begins. We have numerous examples of mythologies of other civilizations and societies. And as most people in antiquity were illiterate, the only form of transmission was orally. The concept of stories were told around campfires probably since language itself was established. They didn't begin when literacy did. I don't have any direct evidence. It's circumstantial induction and a much more reasonable answer than invoking the supernatural.
By the way, one could use the same Kuzari argument for other supernatural mythologies. Consider the Book of Invasions of the Irish, written in the 11th century. It reports many miracles and magics used in the invasion of Ireland by the Milesians ~1000 BCE. Now, the Irish believed the Milesians were their ancestors. And they never came to the Book of Invasions and asked, "Why didn't grandpa ever mention this?" Because they all knew the story already, it was just then in written form.
Now, do you believe all the miraculous happenings in the invasion, like the island turning invisible? It works by the same argument you've offered.