I am so sorry you are going through this. I cannot comment to anything on her addiction as I haven't been there.. But I wanted to say, keep trying to get her to go to counseling about her dad. Not only does the xanax make her numb, but until you face the grief, the shock and numbness from losing someone can last a really long time. Counseling helps. Sometimes it just takes time... sometimes you have to try a few until it clicks and she actually wants to talk about it. I know its hard because you can't force her to do anything, but it's something you should really try to get her to reconsider. The right counselor may be able to get through to her, that she needs to work through what she's been through. My step daughter was 15 when my husband died, I am so thankful that she never turned to addiction, but she has overdosed a couple times trying to end it all. I am "lucky" that she is underage because last year I was able to put her in a treatment center and it was the best thing for her.(Thankfully our medical insurance covered it, its not cheap!) She turns 18 in a couple weeks, she still struggles, but she is more aware of her thoughts, her feelings, etc. and it helps her work through her thoughts instead of acting on them. They explained to me at the treatment center that her cutting was a form of addiction, a way to not deal with the mental pain she was feeling. And she does still struggle with it sometimes. But when she's going to counseling consistently, she does so much better.
Good luck, I really hope you can get through to her. I know how much it hurts to see them go through these things. I know others here will be able to offer more advice on the addiction side of this.