Does the context of the novel justify the graphic descriptions in the opening/first section?

Alice Walker writes a parody of a slave narrative in The Color Purple.  Slave narratives were usually oral confessionals that depicted the horrors of slavery--a form of propaganda used by abolitionist organizations during slavery.  Most were left unedited, depicting graphic physical and sexual abuse, and written in Black English (pigeon English) for authentication.  Of late, many were recorded by the WPA in the 1930s and archived for posterity.  Celie's letters to God are no different, except they depict a slavery within the Black community, within marriage.

The slave narrative had a kind of formula to it.  It began graphically and ended with redemption, usually of the spiritual kind.  Celie's begins with her stepfather's rape and is told by a naive narrator who is disenfranchised chattle.  In this way, Walker shocks the reader into consciousness with emotional blunt force.  It is hard to believe the novel is a comedy based on the first few pages.  Comedies usually move from status-quo to marriage, and I believe Walker goes to extremes to take Celie from extreme low in the beginning to extreme high at the end.  It come across as a fairy tale, a revisionist "Cinderella."

The most famous slave narrative is Frederick Douglass's, in which he recounts his battle royal against his master--a knock-down, drag out fight with Mr. Covey:

This battle with Mr. Covey was the turning-point in my career as a slave. It rekindled the few expiring embers of freedom, and revived within me a sense of my own manhood. It recalled the departed self-confidence, and inspired me again with a determination to be free. The gratification afforded by the triumph was a full compensation for whatever else might follow, even death itself. He only can understand the deep satisfaction which I experienced, who has himself repelled by force the bloody arm of slavery. I felt as I never felt before. It was a glorious resurrection, from the tomb of slavery, to the heaven of freedom. My long-crushed spirit rose, cowardice departed, bold defiance took its place; and I now resolved that, however long I might remain a slave in form, the day had passed forever when I could be a slave in fact. I did not hesitate to let it be known of me, that the white man who expected to succeed in whipping, must also succeed in killing me.

From this time I was never again what might be called fairly whipped, though I remained a slave four years afterwards. I had several fights, but was never whipped.

Alice Walker moves through Celie's narrative in similar fashion.  Attempts to empower her come from Nettie, Shug, and Sophia, but the day must come when she has to stand up to Mr.____ on her own.  Celie's redemption is spiritual, though not from male-centric Christianity; it has a feminist, African slant.  In the end though, the American Dream, by way of the feminine community, save her from sexism.