How does Jane Austen manage the main plot of Pride and Prejudice to suit her purpose in writing?
This is a complex question that presupposes agreement about the nature of the main plot and the nature of Jane Austen's purposes in writing Pride and Prejudice. For the sake of discussion, the main plot can be identified as the action central to the conflict and resolution relevant to the romance between Elizabeth and Mr. Darcy. The conflict, of course, is the pride and prejudice of each that keeps them apart even while being thrown together by circumstances and drawn together by changing emotions.
Jane Austen's purpose in writing might be a little more difficult to pinpoint. If you consider the whole novel, her purpose in writing is a very complex one that entails presenting all the facets of romance and marriage in accord with the Greek and Renaissance literary theory of mimetics, which requires presenting all facets of a value or situation (e.g., marriage) in order to provide divinely inspired instruction on the ideal way to live in regard to that topic. But a discussion under this definition of Austen's purpose wouldn't be in keeping with the idea of a main plot.
So for the sake of discussion, Austen's purpose in regard to the main plot can be identified as the hero and heroine moving from an incorrect view of marriage, themselves and others to a correct view of marriage, themselves and others. With this as the agreed upon purpose in relation to the main plot, we can explore Austen's plot management. The essential components of plot are conflict, complicating (rising) action, climax, falling action and resolution. Austen introduces the roller coaster ride of conflict and complicating action on page one.
Speaking only of Elizabeth, her interest is piqued right at the start, as is true for all five girls, when Mrs. Bennet announces the addition in the neighborhood of two eligible young men at the Nethrefield manor house. With Elizabeth's first encounter with Mr. Darcy at the much anticipated Meryton ball, her interest turns to a different kind of pique when she overhears Mr. Darcy disqualify her from the world of beauty and affableness as a dance partner. For Elizabeth, the mild-mannered war of self-amused retaliation is on.
On Darcy's side, his haughty disinterest is radically altered to subdued admiration (which for Darcy is a radical change) when Elizabeth braves wind and weather and social censure to walk to Netherfield Park to tend to Jane who has fallen ill because of their mother's romantic mechanizations on her behalf. This begins the actions of opposition that continue through the visit to Rosings and Darcy's letter as Darcy becomes secretly more devoted to Elizabeth and she becomes, not so secretly, more disapproving of him.
Austen then manages the plot by the addition of evidence to make Elizabeth begin to see her errors and the truths to which she had been blind, which continues until her visit to Pemberly throws her on the mercy on Darcy's charitable feelings when the horrible news of Lydia's ill-judged and dangerous behavior comes to light. Austen follows this by managing the plot to arrange for Darcy to make retribution for his own mistaken beliefs and conduct, and the resultant troubles, by doing the right and noble thing and becoming the true hero of the story. After the dust from Lydia's actions has cleared and the resolution is underway, Elizabeth has truly become worthy of Darcy, and Darcy has truly become worthy of Elizabeth.