Shes not of a weight to have been a meth addict really. They should have also been able to find that in her blood if she was using it to that degree. While cocaine and things move out of the body pretty quickly, the wounds on her face and neck are pretty fresh and if the wounds/stretch marks were caused by that she would have had to be using quite consistently and recently I'd think.
Also, if you look lower near her neck/shoulder, they begin to look quite round like cigarette burns. Also, if she was a chronic meth user and picking that bad, she should have some other scarring on her face. I have seen plenty of users as I used to work in a recovery center and I have never seen someone damage themselves that extensively all at once. Although its certainly possible.
"My first thought with the shaved head/pubic hair is that the killer is someone who wanted to minimize dna evidence."
That was the thought that came to my mind as well. So then, where did the hair go, along with her hands?