Is the prayer of my late mother accepted, and is she sinful, taking into account that she was illiterate and her recitation of the surahs contained errors that were later corrected?

As long as the mother does not intentionally make mistakes in the surahs she has memorized, she is excused, and her prayers are valid if she does not err in Al-Fatiha and is unable to correct the mistake. This is based on the Prophet's (peace be upon him) saying: "Indeed, Allah has pardoned for my nation their mistakes, their forgetfulness, and what they are coerced into."