Hello, My name isXXXXX am a therapist and have three children of my own, one your daughter's age. I have seen many children in counseling, and dealt with many children who have sleep issues. I was instantly drawn to your question.
Simply said, your daughter needs to have another assessment. I am not at all confident that the symptoms that you described are "nothing". Although it is typical for a child of her age to talk to herself, hearing voices in her head is not. And when they are significant enough to cause sleep issues and emotional distress, this is a problem worth much more investigation. Plus, she herself is concerned about the symptoms and her quality of life is suffering.
No one can tell, unless she has blood work, and a full physical and assessment, but it seems to me that she is having symptoms that are intrusive, frightening and persistent. This a a sign that something is wrong and: No competent therapist would simply dismiss these symptoms as irrelevant or just a sleep issue. (Melatonin, by the way, is metabolized by the body very quickly. 5mg is gone in ten minutes, and it only works if the person is already sleepy and needs a little boost to get there fully.( Your daughter sounds like she needs a real sleep aid, one that is designed to help her fall asleep and stay asleep.
I am quite shocked at the quality of the responses that you have been given by these "profesisonals". If this was my daughter I would insist on a completely new treatment team and new assessment. This is clearly an issue that can be helped, but to say that nothing is wrong...no, not at all. Just her behavior described here is cause for a therapist to look into this further. Hearing voices and seeing ghosts is not a part of normal development and something, either physical or mental is driving it.
I do feel she can be helped, for certain, but she needs a proper assessment and diagnosis. I often encourage parents in situations like this to call the local children's hospital to ask for a referral to who they feel is the best child specialist in your area. Your daughter can be assisted, but she needs a second opinion, and not just a sleep study. Be encouraged, with the right assessment she can be helped. Steven