If she doesn't think she needs help, this will only make her even more "everybody is against me". It would not make her realize she's been wrong all along.

Maybe she needs to learn the hard way. You cannot force help on people, or force them to change ways. And there seems to be much more problems than her being depressed. It sorta sounds you are making excuses for her and you want to believe that if she's sectioned, she will magically become the person you always wanted her to be.

Nah. Some people are problematic all their lifes. Some change, but hardly ever it's through being sectioned.