I'm going to address the self harm part first as it will speak to the other questions you have.

When a person self harms, the reason is most often because there is a lack of healthy coping skills to address whatever stress is going on. In this instance, your mother. To blame her for your self harming, no matter how difficult and challenging she may be, is a perspective you need to change. No one is responsible for our behavior except for ourselves.   Address the internal coping first.

With regard to whether or not you can visualize changes in her. Of course you can!  But manifesting this will be no different than manifesting other things and the instances in which there is a POI invloved.

This process, is one in which you go into yourself. It's building yourself up to be the best version of yourself that you can. That's what the meditation is about.  Yes, we visualize the things we desire and the things we want to hear but the key is in letting go and focussing on yourself and being the best person you can be.   

I am also very sorry the situation is challenging. I personally believe we are capable of changing whatever it is that we desire.

I say this with love, you've got some internal work to do.