Well, here's the thing about 522. We don't have any evidence that it ever actually was performed. Though dated June 14th, 1787, the piece didn't appear in print until the beginning of the 19th century. A full ten years after Mozart had died. (This is attested in Keefe's Mozart in Vienna, p 493)

I honestly think anyone who tries to say that "Mozart didn't let us know his intentions with this piece" and trying to use that to suggest that Mozart had any sort of lofty intentions are a bit too caught up in a vision of Mozart as some divine genius who people took super seriously all the time. He wasn't. He's very clearly taking the piss. It's right there in the title (Mozart's own). How much clearer do you want the man to be?

You gotta understand that this is pre-Beethoven. The notion that audiences are to sit in silent reverence as they bask in the light of a genius is really a Romantic thing. It's not really how we should understand Mozart and audience reactions to him. This is especially true of divertimenti, which are designed to be light entertainment accompanying, say, a small dinner party amongst friends and guests. No one walked in, dimmed the lights, and shushed people who were too noisy: people were drinking, eating, conversing, smoking, playing cards, etc. Think less a modern day concert hall, and more like a cover band playing at a bar.

So like this stuff

Did they enjoy the way it sounded?

If they enjoyed how it sounded, did this piss off snobby musicians who did things by the book?

Was it a kind of showboaty, "Because I'm Mozart, I can make all these 'mistakes' and still make it sound good" type deal? Did people recognize it as such?

Is pretty far off base, I think. Mozart certainly enjoyed a pretty successful and renowned career as a composer, there's no doubt about that. But in no way was he regarded as someone who could do no wrong. Plenty of people criticized Mozart's music, with Dittersdorf, for instsnce claiming that "because of their unrelenting extreme artfulness [Mozart's music js] not everyone's purchase." (Translated in Gjerdingen, Music in the Galant Style, p. 437). This was perhaps the most common complaint: Mozart could clearly write awesome music, but man, it can get pretty weird at times.

And the reason why this criticism could resonate with eighteenth century audiences is precisely the reason why the intent of 522, assuming it even was heard by anyone,, would have been clear as day. The 18th century aesthetic was an aristocratic one guided by ideals such as tastefulness and judiciousness. Being weird af is in no way a positive thing: doing wild musical shit doesn't mean you are a boundary-pushing genius, it means you have bad taste. And 522 is bad taste incarnate. Given an audience that actually likes their musical language, and for whom the ability to assess the merits and flaws of a piece of music was an essential value, there can be absolutely no doubt that an audience would have understood that this is supposed to be bad music. 522 is essentially a fart joke: blatant, brash, and lacking any sort of subtlety.

Likewise, making fun of "snobby musicians who did things by the book" is also a little off-base. While, yes, Mozart could certainly be critical of aristocrats, there is no question that he aspired to be one. The notion of Mozart as a "composer of the people" is pretty much a fiction. Mozart was a snob and worked for snobs pretty much his whole life. He simply also liked super crude humor. This is the kind of guy who, were he around today, would absolutely be trying to schmooze his way around fancy cocktail parties, but whose favorite movie is probably Jackass.

Doing things "by the book" is not a bad thing from an eighteenth-century perspective. People like the book! Pretty much everyone is happy with the book! The book makes good music! Making good art is not really about inventing new sounds, but rather about taking the resources you have (and that your audience understands) and combining them in ways that are compelling.