Whether she is about to turn 18 does not matter as the authorities will have it on record and she can then be transferred to the adult services.

You can also report anonymously if it concerns you. The protection services are the experts to go to.

The child is displaying severe mental distress as of the symptoms you describe, this can lead to psychosis. She needs help and this can be a sign of abuse. Its not always about physical evidence such as bruising but also psychological evidence here. You've documented well here the verbal and emotional abuse taking place. Threatening to lock up a child is not normal and taunting her when she's suicidal is definitely abuse.

Please report it, put it in their hands, you'll be glad you did and the child can get treatment.