I am very tired of stupid people making this stupid argument:
The fact that every Jew in the world today beleives in matan Torah proves it happened. Because where else would they have gotten the idea?
The answer: Ok, listen carefully. There are roughly 2 billion people on this planet who think Jesus was god (5 billion + 1 if you count the prop. of Cross Currents) Is that proof of anything other than a mass delusion?
Lots of crazy ideas have been accepted over the centuries. Million of Celts believed in fairies. Irishmen believed in Leperchaums. Polish fat women believed in all manners of demons, ghosts and witches. How did these ideas become, for a time, so widly accepted?
The very best we can say, citing the writings of Josphous and his coeivals, is that some people in the year zero thought their ancestors had experienced a revelation. What does that tell us about the other Jews living at that time. Nothing. What does it tell us about any of their ancestors? Also nothing.
I agree you can probably prove to a disinterested party's satisfaction that some Jews living around the year zero had embraced a mighty strange idea.
It doesn't follow from this, however, that the mighty strange idea was true.