Like I said, you're projecting a picture of Native Americans as simpletons with no technological advancements. Europe, at least at this point, was no more advanced than just about any other part of the world. If you take away shipbuilding and metallurgy, two fields where Native Americans had little interest, there's just about nothing that the Europeans were indisputably more advanced at.
I actually take exception to this at no point have I called or implied that the Americans were simpletons, the technological advancement of a society is not an indicator of the inherent intelligence of it's members. The fact of the matter is that at this point in time (1492) the European countries were more advanced in a number of critical fields such as writing, metallurgy and siege craft that gave a them a decisive advantage over the American nations when it came to warfare.
It's not a weak argument. It's an observation made by many modern historians and even by the people who were involved in these events.
Yes explaining something away as luck is a poor argument it's also lazy reasoning. Luck plays a role in all human endeavor, you can't simply chock things up to luck and expect people to be satisfied with the explanation. Luck caused the Aztecs to initially welcome Cortes it didn't cause the Aztec tributary states to join Cortes, that was caused by demonstrations of Spanish military power and weakness in the tributary system. Also to your point that the Spanish probably wouldn't have come back soon, reports of the gold that Aztecs had are what brought Cortes, if he had failed but come close it's almost certain others would have followed especially considering how much the Spanish throne needed money to finance it's wars in Europe.
As a side note Pizarro did read and use reports by Cortes and admitted that his capture of Atahualpa was inspired by Cortes taking Moctezuma prisoner.