This is a funny thread for me as an Ami.

Yes, there's a difference between aww and n'aww. And there are many shades of difference.

For your context, maybe:
Boy: "Ohne Dich ist mein Leben total langweilig."
Girl: "Nee, das kaufe ich Dir nicht so (ganz) ab."

Again, it depends on context.

To add to #1, search the forums for threads about "naw" and "nah"--those are the basic ways of spelling this variant of "no."