A little edited to make more sense (thanks for the comments).
Another edit: I am horrible at explaining these things in my terms; see the comments for elucidation.
I am becoming more convinced that な is a modified form of だ, or visa versa. And の mixes into things too.
If it is after a na-adjective, but not before a noun it is expressed as da (奇麗だ vs. 奇麗な顔). In front of other copulas, da hides. It's unstable like that.
Then I came to feel a few minutes ago that だ becomes な／の in a slight prettying up of statements and explanations. Consider: 〜なの could be from (a hypothetical)〜だの and in actual reality we do know that だから changes to (な)ので.
I don't know. Maybe this is a crazy theory. It only really helps for remembering anyways. If you remember things in a circuitous manner like me.
you need something for those glazed over eyes?