it's theoretically possible that the Aztecs could have made advances in shipbuilding that took Eurasia well over a thousand years
Not unless they had somewhere to go (people don't build ships because they're exciting, the build them to ease trade.) However that's not what's happening. What we have here is what I like to call "The X-Com scenario"- a large group of people are invaded by a small but highly advanced group, in the process tides are turned and technology is scavenged (this is also the plot of the WorldWar series, for example).
but it's also possible that a meteor falls and destroys Spain
Not all of Spain, but, sure, a major city destroyed by some Tunguska analog, would alter history.
the Cathars invent the steam engine
Possible but irrelevant- steam engines are useless without coal. Trying to feed a steam-mill with lumber is less cost effective then just using a regular windmill.
[Cathars] defeat the Albigensian crusade with Gatling guns.
Where as this is out of the question entirely. You need so much technical and
social change to pull this off, that to call the resulting society "Cathars" is at the very least misleading. Otherwise you will be left with one (or a couple) Gatling guns (designed by some ahead-of-his-time genius, already burned at the stake) and no ammo or personnel to run it after a few years.
Besides, Maxim Guns, while just a generation away from Gatlings, were a lot more devastating. If it can conquer Africa, it can beat back a bunch of frog eating surrender monkeys. But again- a Maxim gun without supply and production chains is just a 100 pound paperweight (monument?).