1. No.
2 & 3. If the police have reason to think you pose a risk of serious harm to yourself or others they may take you into custody and transport you to a mental facility for evaluation and observation. What happens from there depends on what the medical staff there determines about your condition. They would not simply take you home. The police may also, of course, arrest you and detain you if they have probable cause to believe that you committed some crime.
4. Once you turn 18, the police cannot just take you home. Doesn't matter the reason, with one exception. That exception would be if you were declared to be incompetent by a court and the court appointed your parents as your guardians. You are an adult at 18. Your parents, apart from that one exception, no longer control what you do.