The problem with all this is, as others have said, geography. Pure and simple.
Even if the Papal Bull had granted ALL of the Western Hemisphere to Spain (which I don't see Ferdinand and Isabella agreeing to, as it could cost them more territories in Africa), the Spaniards would not have been able to explore, settle, and defend the whole of present day Latin America by themselves. If nothing else, the Spanish Reconquista of Portugal would probably mean that the Dutch will manage to keep Recife'.
Even if some kind of Brazilian Napoleon managed to conquer present day Argentina, Uruguay, Paraguay, and Bolivia, only Argentina would be a money paying operation. The rest of South America simply has too much Andes Mountains and Amazonian Jungle in the way for the Brazilians to reach, much less conquer and exploit.
What do the Brazilians get for taking the Andes, even if they can? Nothing on the other side except the Pacific Ocean. They get a West Coast, but it's in the South East Pacific. Not much down there. And then there is the little matter of navigating the Straits of Magellen, or Cape Horn!
It's not like crossing the Continental Divide in the Rocky Mountains, which for the US meant reaching and exploiting the riches of the Pacific Northwest and California. Not to mention the mineral treasures all over the Rockies.
One only has to imagine the difficulties of trying to live in a place like the Upper Amazoco Rain Forest compared to the plains of Nebraska.