> On top of being what enabled them to form alliances, it also begs another question. If it was primarily these masses that overwhelmed the Aztecs (and I don't disagree) then what need did they have of outsiders
You presuppose they needed the outsiders. The Aztec empire was not some ancient, unwavering fact of life which the native mesoamericans could not conceive of changing. It only lasted 100 years. It is entirely possible that the rebellion was going to happen anyways in the next few years or decades and the arrival of the Spanish was just the butterfly flapping its wings that happened to trigger it at that moment.
> justified the Spaniards being deemed leaders.
The native allies of the conquistadors installed the Spanish to replace the Aztecs in exchange for a lower tax rate and increased independence. They did not accept spanish leadership, they just preferred empowering a neutral third party than any particular longstanding rival. Spanish domination came later after smallpox decimated the native population and intermarriages with the pre-existing elite meant there was no native power structure left to stand up to them.