You can only be required to stay for mandatory overtime by the employer (manager). Managers are PTMs, Program Directors, Supervisors and the designated “Administrator on Call” for the hospital.
A CRN or charge nurse cannot be required nor should they agree to assume the duties of an out-of-scope (not within the union) manager. The employer can direct a CRN or charge nurse to ask for volunteers for overtime, to prepare a list of nurses working that day, and/or to identify those nurses who previously had mandatory overtime, but they cannot direct a CRN or charge nurse (nor should a CRN or charge nurse agree) to force a co-worker to do mandatory overtime. A CRN or charge nurse who does this may find themself in a situation where the union files a grievance saying that mandatory overtime should not have been imposed. In that case we would then in fact be saying that the CRN/charge nurse did something incorrect.
A nurse being denied the right to leave the facility should only take that direction from a manager.