Lox - I grew up in a family with a substance abuse issue. My parent eventually recovered and things got healed properly. However, since I was in my midteens when the recovery took place, I was fully aware of just how close to total annhilation our family came. I need to say some things to you as the daughter of an 'addict'

You are the sober parent.
You are the one in a postion to protect your child
You are the only one right now who can take certain measures to protect her
You are not married to her mother
As a result, what happens to her mother should be secondary in your concerns
Your child's fuuture mental health depends on what you do now to protect her NOW, not what happens to her mother now.

I know you worry about how she will be impacted if her mother falls even further. However, That is something you don't currently have the luxury of obsessing about. Your ex-girlfriend is a threat to your daughters life, health and emotional wellbeing. You must gather the strength to DISPASSIONATELY do whatever it takes to keep this woman away from your daughter until such time as she has recovered and has been sober for a substantial period of time.

Your ex is a nurse and under the influence of susbtances that greatly impair her judgement. She could kill patients. it is appropriate for you to disclose her condition to her superiors if you have the documentation to prove your accusations. Exposing her substance abuse is appropriate and may help you get the court protection your daughter needs.

As for the visitations - if she has legal right to visitation, you must see about getting those visits ordered to be supervised. The last incident with her taking her to a dealer and then leaving her in a dirty diaper.. should be enough to get an emergency order.
Here in the USA, a father can get a mother's parental rights TERMINATED for such behavior. I don't know what your legal adviser says about such things, bu6t I do hope you are confiding in him/her all these things.

I know she is the mother of your child and you aren't a monster and therefore you are hoping she will have some sort of relationship with the girl. That's good. However, if I could have been heard when my life was in danger from my parent's addiction (and it was in danger frequently) I would have said:

"Mummy/Daddy/Whomever, please for godsake make this insanity stop. Do not leave me alone with this parent! Please tell whomever you have to what is going on: Grandmother/father, teachers, police, clergy, social workers, whomever. I don't care. Just keep me safe. Sure I love {Parent}, but they frighten me. They do things in my presence I shouldn't have to see and expose me to dangers no child should have to endure. These experiences confuse me and make me afraid. All I want is for somebody to stand up and be the adult. If I have to choose between being safe and never seeing {parent} again, I choose never seeing {parent}. If you love me, please protect me and don't make me go with {parent} again until {parent} is sober. Although I am young, these scary encounters affect me. Please just let me be a child. Do what you must and when I am older, I will be so grateful"

I hope that's not to harsh. If I had been able to speak up when I was very little, that is what i would have said.