To what extent was mayella free to speak the truth? in to kill a mocking bird

Technically since Mayella is under oath while she is testifying in court, she should be speaking only the truth and nothing but the truth. However, she does not feel completely free to speak the truth. 
If she were to speak the truth and tell the court that her father is the one who beat her, it is very likely that she would receive another beating. Fearing for her safety is one obstacle that keeps her from telling the whole truth.
Societal rules are also keeping her from speaking the truth. If Mayella were to confess that she is the one who made advances towards Tom, she would be further shunned by the townspeople. During the trial, she is gaining a lot of attention. Normally, she is rarely seen by the people in town, and when she is rarely is she acknowledged. This trial gives her attention. By revealing that she is not as innocent as she appears to be, she would lose her stage.