I don't believe you have a case against this t.
You are responsible for your choices and she gave you the option of the waiting room or home. If you had chose the waiting room and decided to leave anyway what could she have done about it?
Therapist do not have the power that she could have kept you in her office. If you had insisted on leaving she would have to let you go anyway.
The fact that you had shown up to the session (you could have attempted suicide beforehand) and had agreed to a hospital admission shows you had some insight and were taking steps to keep yourself safe...but made an impulsive decision and won't take responsibility for it.
If she had let you leave without a plan and without checking in then this would be a different case.
I feel quite sorry for her being put through this.We give our therapists so much power we forget we have our own and they are not responsible for us or our actions outside of the sessions...thats our job.