While I feel you make some good points, please recall that I was specifically
not proposing a single state but a
supranational union of states in Hispanic America.
As a side note, the Indian Subcontinent was unified long before the Muġal Empire, with the Maurya and later Gupta Empires in Antiquity and the Early Middle Ages, respectively.
If you want another example of geography alongside the Inka Empire and India, just look at the USA. In many ways the US had more disadvantages than Hispanic America: The great majority of what is now the US was, at the time of the Paris Peace Conference, either inhabited by a great number of amerindian nations speaking more diverse languages and than even India while maintaining little cultural similarity with nations and peoples outside their cultural area: Thus, while a Maidu person may have some cultural similarities with a contemporary Pomo person -a sedentary hunter-gatherer lifestyle, a shared Kuksu Religion (albeit with some minor differences, of course), for example, neither the Miwok nor the Pomo person would have much in common with, nor know much about, a contemporary Ktunaxa person of the Interior Plateau or Nuxálk person of the Northwest Coast, and likewise visa versa and so on.
Also, note that travel to California, until the Transcontinental Railroad, was a great difficulty, requiring a variety of sea and dangerous land routes. Yet California is not Russian, not Mexican nor is it Canadian but rather part of the United States of America. Finally, even Texas, which
does have pretty easy access to the Eastern Seaboard, took quite a while and a lot off bloodshed and effort to become part of the United States of America. And the United States didn't just settle for Supranational Union, and dumped the idea of a loose confederation early on, and has endured a Civil War, the Great Depression and even maintains an impressive amount of ideological unity (AKA patriotism, belief in the "American Dream", &c.) more or less throughout its territory.
In conclusion, do not forget that it would have been a great deal harder to travel from Quito to Cochabamba in 1500 than it would have been to travel from Mexico City to Cusco in 1800.