Outside of an instructional situation (teacher, parent, etc.), I think it would be extremely obnoxious to correct that sort of non-standard English phrasing.
I'd agree. There are few cases where it's acceptable to correct someone else's language.
I disagree with the notion that a double negative must logically mean a positive.
(Disclaimer: I'm by no means an expert in logic.) It depends on what your axioms are. Two negatives
can mean a positive, and in standard English they often do, but there's usually contrastive stress applied: "I didn't
not do my homework." But nonstandard varieties that use multiple negation don't usually do this—they just distribute the negation, without extra stress, throughout the sentence, like "I didn't do no homework." The first sentence might be represented as something like –(–(do homework)), while the second might be (–do) (–homework). There's no reason to assume that the first negative
has to have scope over the whole phrase, thus cancelling out the second.