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Let me understand this. Your mother is damaging property, stalking people and threatening arson, an action which could kill her own daughter. The violence is escalating. And you're writing letters to us?
You can talk to mental health professionals and get support and look at treatment options, but you need to do more - and fast.
Call the ex-boyfriend and try to develop a timeline and more details about your mother's deterioration into insanity. He also might know the medical people who have been involved in her treatment, or lack of it.
Encourage the ex-boyfriend and his new lady to file police reports, just to get the behavior documented.
Talk to the police and start a file on her. Talk to neighbors, warn them and ask for help in monitoring her behavior. Establish the pattern of events and the escalation. Give the police the tools they need so that if she acts again, they can order her held for psychiatric evaluation. Get Social Services involved and see whether she can be labeled a vulnerable adult.
You may not be able to have your mother treated involuntarily, but the police might.
Don't wait until she kills someone and you have to explain why you didn't do more to prevent it.