More than slightly unrealistic, but remember that in 1492 Tenochtitlan was the largest city in the world, and the Aztec Empire the largest and richest, far larger than Spain.
Yes. in the story he connected with first one, than another of large empires. Married local leaders daughter. Had a awakening of his own....and took it from there. Gave them technology and knowledge they did not have while using the best of what they had. Unrealistic? Oh yeah. But hey, it would not be entertaining science fiction if it were not somewhat unrealistic. I believe the driving force to his returning was his son who was left in the care of a monestary while he was away.