Also, if the tradition was so “phenomenally reliable”, why is the story so inaccurate? For example, it says that in 1531 Juan Diego is running around having dealings with Bishop Zumarraga, who wasn’t even in Mexico in 1531. It seems to me, even if they were transmitting the story faithfully until it was published, at best, they were transmitting a demonstrably unreliable account.
With the title of Protector of the Indians and bishop-elect (before formal consecration), Juan de Zumárraga arrived in Mexico on December 6, 1528. He is listed in the ecclesiatical chronicles as Archbishop of Mexico 1528-1548.
A very old and battered partial (16 page) Nahautl manuscript copy of the Nican Mopohua, dating c. 1556, can be found at the Public Library of New York.
The Nahuatl document I mentioned before, the Inin huey tlamahuiçoltzin, is kept at The National Library of México.