Who says that we can only get here in 1492?

The capability to get the Americas was there since... well the Vikings to be honest. It was the will that lacked.
Sure it got easier with the compass and other inventions, but sailors were not bad at knowing which way was what even without them.

So I think it is going to be a case of certain requirements are filled for the chance to go to America is opened.
And I cincerely doubt the Aztecs will be playable in the normal sense. And I doubt that their lands are part of the main map. As they say, it is an extension of the map. That sounds like, a ship that goes [here] will end up in another map.