3) About the only way to slow this down (unlikely to completely stop) might be for the Church to ban westward travel on the Atlantic
Why? Apart from this, I agreee with all your other points.
If Columbus fails, it fairly certin that whoever gets to Mexico first would probably come after Cortez did in OTL. Since the Aztec Empire was already failing and Cortez needed a crazy amount of stuff to go his way in order for him to be successful (and he was acting against orders to begin with), is there a realistic chance that the Meso-Americans are able to resist being conquered, at least partially?