How can a female preacher achieve da'wah accomplishments and have devotion in her prayer, while committing the sin of backbiting and harshly criticizing her parents in front of people?

Perfection belongs to Allah alone, and all humans are fallible, and the best of those who err are those who repent. This is not meant to encourage committing sins, but rather to show Allah's love for those who repent and His acceptance of their repentance.

A caller to Islam (Da'ee) must be an example, and their actions should not contradict their words. They must also repent to Allah from backbiting people. Allah Almighty says: "O you who have believed, why do you say what you do not do? Great is hatred in the sight of Allah that you say what you do not do" [As-Saff: 2-3], and His saying, the Almighty: "Do you order righteousness of the people and forget yourselves while you recite the Scripture? Then will you not reason?" {Al-Baqarah: 44}.