Mummypig1973 I am really sorry but you are indeed totally focusing on the wrong issue.
Whoever contacted the school almost certainly did so because they either saw or heard something or were told about something by their child. Whether this was actually accurately about your child or not, I don't know. The idea that anyone would contact the school to be malicious about this issue seems very unlikely.
If my child (aged 12) told me something about another child I would tell the parent if I knew them well, if not I would tell the school. I think it is totally appropriate for the person who told this to be and remain anonymous. It would be terrible if anyone else decided not to give 'information or act on a hunch' to the school because they felt it could not be treated anonymously.
In terms of your child, they may be fine, or they not. My daughter has expressed suicidal thoughts, they came out of relatively no where and we have a referral to CAMHS. It is dreadfully scary but it is nothing to be ashamed of. I have done nothing wrong as a parent, and my child has done nothing wrong as a child.
I don't talk to all and sundry about it but I have shared with certain good friends, especially ones who have had issues with their kids and will understand. I have certainly shared with her teachers/school nurse etc because it is relevant for them to know what is going on.
What I find worrying about your attitude is that you seen very certain you are right about this, very certain you want to know who contacted the school - I am wondering what you will do with the information, would you approach them?
Plus the very long session of discussing of this issue (over an hour) seems like a bad way to get the truth out of a child and IMHO it is better to talk to them casually over an activity, allowing lots of chances for them to say things without much prompting.
Anyway, I hope your daughter is totally fine, I hope you will start to focus on what may have prompted this (aside from malice) and that all will be well.
CAMHS have spoken to my child and they know she does not have a plan, but she does sometimes feel life is not worth living. I had similar thoughts at 15, I think it is quite normal, even in my twenties when I was on my period I felt so unhappy, took me years to make the link to periods.