How can I atone for my mistreatment of an Indonesian maid whom I insulted, and who has returned to her country, and of whom I know nothing, given my fear of Allah's punishment?

If you have wronged that woman and insulted her, you must repent to Allah and seek His forgiveness, for Allah forgives all sins for those who repent. If you are able to ask for her forgiveness, you must do so as a condition for the validity of your repentance. If you are unable to do so, then increase your seeking of forgiveness and supplication for her.