Is she sinful for her testimony in court to which the judge added a phrase she was not sure of, knowing that she swore on the Mushaf? And is there a way to clear her conscience?

If you testify to what you know without alteration, addition, or omission, then you have fulfilled your duty and bear no responsibility. And if you alert to your lack of knowledge regarding its expenditure, then you are absolved, and the sin falls upon whoever adds to or alters the testimony.