I think even Montefiore himself would probably agree with your assessment, as he later wrote a "prequel" of sorts called Young Stalin. This covers more of the war and lead up to revolution. Perhaps it would be better to read that first? I'm in the opposite position as I've not yet read The Court of the Red Tsar.

I'm looking for a book on the history of Buddhism. Is there anything like what Diarmaid MacCulloch has written regarding Christianity?

I'm particularly interested in how Buddhism has changed as it has been adopted in new countries/cultures. Ideally it would be written for a lay and secular audience.

Is there such a book?