I really should find time somewhere to listen to both this talk and the one you posted the other day.
I have taught the material a good number of times, and follow the argument with regards to revelation to a whole versus revelation to a few as you present it in the post from Exodus 20:1-18—the problem being that in 20:19 the people chicken out (understandably, no doubt I would have done the same) and request that Moses act as an intermediary—and what Moses sees as an intermediary is more than what Moses is told to tell the people, which in turn is usually more than what Moses actually tells the people at the time.
But I imagine that the talk addresses this, at least in part.
Moses spent several lengthy periods in supernatural communion with God. He then spent forty years teaching what he had been taught.
The Children of Israel camped for long periods in relative isolation free of worry about food and water. How do we suppose they spent their time?
It was only until about a month before his death that Moses wrote the Cliffs Notes that became the Five Books of Moses. Torah is not a book, it is a set of teachings.