Why does Mary Grace deliver her message to Mrs. Turpin instead of one of the other characters in the waiting room?
Short Answer: Mary Grace's message to "Go back to hell where you came from, you old wart hog" strikes the "target" of Mrs. Turpin because this woman is such a self-righteous hypocrite.
Mary Grace, whose name signifies her role in the narrative, has listened long enough to Mrs. Turpin to realize that although this woman has been supposedly "saved," she has an uncharitable perspective and feels herself the superior of blacks and poorer whites and has been speaking to Mary Grace's mother and others in the waiting room of a doctor in her self-righteous tones. So in disgust, Mary Grace hurls the book she has been reading at Mrs. Turpin, then falls to the floor, succumbing to a seizure.
The girl's eyes seemed lit all of a sudden with a peculiar light, an unnatural light like night road signs give.
Mrs. Turpin, who feels that the "girl did know her" because of the light in her eyes, bends down and asks her "What you got to say to me?" It is then that Mary Grace provides Mrs. Turpin the "revelation" that she anticipates, only what Mrs. Turpin hears is not what she has expected. Nevertheless, she has been "singled out for the message" and she ponders it until this revelation alters her perceptions as "a visionary light settled in her eyes." Mrs. Turpin envisions herself and her husband and people like her bringing up the rear in the procession to Heaven.