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?
なぬ is an exclamation to tell surprise, like "No way!" It's probably related to なに, 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.
ぬぬ isn't a meaningful word, instead
a way to spell out grunt, or the sound you utter when you're too shocked to answer anything back. = ぐぬぬ, むむむ, うぐぐ etc.
a way to spell out nasal sound you utter to yourself to show a light doubt. "uh?", "huh?" = ぬ？, ん？, うん？