"Historians argue that China and Tibet remained two separate units" 

Hi there. Are you sure that sentence (which you added to Amdo) appears in the source cited? (The Story of Tibet: Conversations with the Dalai Lama, by Thomas Laird, Grove Press, 2006, ISBN 0802118275)

I couldn't find it in there. Could you give me a pinpoint (i.e. page) reference?

The same sentence appears in History of Tibet, where it is tagged "Citation needed". If you can give me a pinpoint citation for the sentence, it should also be added to that article.  (Talk)