What exactly separates the grammar of “I (and Dave) am going” vs “Dave (and I) is going”?
The same thing that seperates the first one from “going Dave am I and”. Word order matters.
the order of the coordinations is a matter of style, not grammar.
Categorically false
Therefore, the only “wrong” grammar is constructions that natives wouldn’t use
Wrong again. Plenty of native speakers of just about every single language there is have atrocious grammar. Native ≠ correct grammar.
It’s not wrong, merely a different register of speaking
It’s wrong when it comes to grammar. Whether or not gramatically incorrect colloquial speech is acceptable and sometimes even preferable to being gramatically correct (it is in most cases, but in some it can be very grating) is a different matter entirely.
Accepting only prestige dialects as “correct” grammar
Nobody said anything about dialect
people continue to think things like "black people don’t speak right
As I mentioned earlier, colloquial speech can be as good as or better than gramatically correct speech in some cases.
You’re mixing up a whole lot of things that aren’t grammar and just generally being wildly wrong all over the place.