When you look at documents historically, then you have to consider the historic context. Indian Mahayana sutras are quite elaborate, perhaps a bit overbearing to the modern reader. But there are a lot of things we do not take into account. Such elaboration, for example, the introductions which just seem to go on and on and on, naming hundreds of Buddhas and reckoning countless bodhisattvas and such, it seems right over the top. But in its day, this would be considered appropriate for the importance of the texts. It is appropriate in proportion to the value of the teachings. You can see the same thing with things today, if it isn't given a whole lot of pomp, people might not pay any attention to it. Think of how much build-up a new movie gets, or how much money and effort is put into a wedding. This is not exactly the same thing, but the point is to consider the whole context.
As far as the validity of the teachings goes, nobody can prove that about any of the Buddhist teachings. The Theravada teachings (Pali Canon) were not written down until about a century after the teachings were given, and then, some 5,000 miles south of where the Buddha gave them, and in a language he didn't use. But consider this; Who was it who first rubbed two sticks together to make fire? Nobody knows. Yet, if you do that today and produce fire, the fire is just as valid as the fire produced in prehistoric times. so, the validity of the teachings lies not in historical documentation, but in being able to test out those teachings in the present day and replicate the results. They are valid because you can test them out for yourself.
Profile Picture: "The Foaming Monk"
The Chinese characters are Fo (buddha) and Ming (bright). The image is of a student of Buddhism, who, imagining himself to be a monk, and not understanding the true meaning of the words takes the sound of the words literally. Likewise, People on web forums sometime seem to be foaming at the mouth. Original painting by P.Volker /used by permission.