Are なぬand ぬぬ like slangy/cute ways to say なに？ When I say them out loud, it makes sense like someone in english saying "wuuut" instead of "what?", but that mostly applies to written casual conversation I guess. Anyway is this correct or is it something else?

is an exclamation to tell surprise, like "No way!" It's probably related to , but has lost most of its interrogative nuance. It doesn't sound particularly cute, but a bit old-fashioned and perhaps dialectal, thus mostly used by a modern speaker to add some comical tone rather than a serious surprise.

isn't a meaningful word, instead

1. a way to spell out grunt, or the sound you utter when you're too shocked to answer anything back. = , , etc.

2. a way to spell out nasal sound you utter to yourself to show a light doubt. "uh?", "huh?" = , ,