Now, this is exactly the type of document we would like to have in our possession to examine ... But don't forget that many documents got lost during exile and the pertinent ones, if they exist, are not going to be handed to us.
They say that for many parts of the world and many historic cycles the Vatican Libraries hold the type of secrets you are referring to.
Unhappily we don't even have such a slight hope as knowing that the documents are in a specific place and that we might be able to locate them some day...
Imagine, friend, that things that happened if front of journalists, that are filmed and stored, are later on denied. And nobody cares. How are we going to find documents to disprove fake documents?
For example, the Colgate University demonstration happened, as it were, hours ago. Everybody can see the video, can see the amount of people in attendance, can see the Dalai Lama's spokesperson talking to them.
The other day, this same spokesperson for the Dalai Lama was asked a question about this demonstration in a radio in Asia. Do you know what he said? Oh, yes, yes, there were some people there, like 4 Tibetans and 20 Westerners.
...
So, if what we know about the behaviour of a person contradicts documents about the same person ... well, we have the power of inference on our side.
My suggestion is:
Let's tell the world the truth, that everything indicates that this letter is a falsity introduced in a true document, that nothing in the behaviour of Pabongkapa shows the possibility of it being true.
Let's also tell the world that the Thirteen Dalai Lama was a follower of Dorje Shugden's advice for such huge matters as renovating stupas and the Potala itself in order to create a protection for Tibet. Which allows us to infer with excellent reason that the Thirteenth Dalai Lama's inimical mind against the Protector might be another lie or exageration or twisting the meaning of long ago occurrences.