sleipner you are right that dogs need discipline in their lives and they will give respect without fail once they believe that the leader is fair, understanding and above all strongly giving the right signals.
If your dog is still barking when she shouldn't - and even if you believe that she knows she shouldn't - then you are not giving her the signals that she needs to enable her to accept you as the leader and live by your rules.
Swatting her on the butt is a very strong signal to any dog - it makes not a jot of difference which one as they all understand the same signals - that you feel strong in your position only by using physical means. The fact that the physical is ineffective strengthens the dog's view that you are a weak and fallible leader and not to be trusted with the welfare of the pack. She has no need or reason to pay attention to discipline from such a leader as by doing so she could put herself at risk (in her eyes).
However close your relationship with the dog is - she is still a dog and will understand as a dog. She is incapable of anything else.

She is reacting to weak leadership by taking it on herself to look out for the pack.

I'm afraid to stop her barking you will have to have a change of attitude or find a trainer that shares your views. There are methods that will work and can be explained in different ways but when you analyse them it all comes back to getting your dog's respect. Sadly this is misinterpreted by many as gained when they get a fear reaction from the dog.

Good luck, let us know how you are getting on - she sounds like a really great dog.