Mansfield Park is an enormously complicated novel, even by the standards of Jane Austen, who creates characters and situations of unusual complexity in all her novels. Like other Austen novels, this one is concerned with a young woman trying to find her place in the social order. Fanny comes from a poor family but is being raised by her rich aunt and uncle. She prefigures the orphans of later Victorian novels in her separation from her parents, who will not be the primary determinants of her eventual status. Like other Austen heroines, Fanny will, in part, determine her status by marrying. Since women could not enter the professions, marriage was the only way, in the nineteenth century, to ascend or descend the social ladder. Fanny's mother has fallen downwards quite a bit through her own marriage to a sailor who turns out to be a drunk; her aunt Lady Bertram and her cousin Maria, on the other hand, do fairly well by marrying. While the marriages of others have been formulated based on beauty and family connections, Fanny is to "earn" a marriage partner based on her character. Virtue is definitely rewarded in this world, and it is the primary determinant of an individual's eventual fate.
Mansfield Park is interested in far more than just the settling of social status, though. In part, it takes up the age-old debate over whether "nature"--one's innate qualities--or "nurture"--the environment in which one is raised--is the primary determinant of character. Fanny and her siblings, and Mary and Henry Crawford, are ambiguous figures in this regard; all of them are shuttled between different households growing up, and it is never clear whether it is their underlying personalities or their situations that have made them what they are. This makes for much interesting debate in the novel, particularly as Edmund struggles with his feelings for Mary and tries to justify her behavior. The idea of education is a part of this debate: can people change? Clearly, by the end of the novel, both Sir Thomas and Edmund have learned something, and the role Edmund has played in forming Fanny's mind (and, to a lesser extent, the influence Fanny has exerted over her sister Susan) speaks to the capacity of some individuals to change for the better. Others, like Maria and Henry, never seem to learn. Urban and rural settings are used as backdrops for this debate, with the suggestion being made that city life promotes vice and inhibits one's moral development, while growing up in a country house exposes a child to all that is good. The Bertram daughters and their oldest brother complicate this, though.
This may be because country life is not free from corruption. This is Jane Austen's most socially-aware novel. Sir Thomas is absent for nearly a third of the novel, tending to his business interests in the Caribbean. He is a slaveholder, and this fact is directly addressed when Fanny asks him about the slave trade. It is while he is gone that the family goes astray, and while this suggests the need for paternal authority, it also implies that his affairs--trafficking in humans--are a moral liability. In general, Austen is very aware of the world around her in this novel. She depicts urban poverty in her portrait of Fanny's parents' home, and she uses the gossip sheets and other forms of then-modern media to further her plot. This is also Austen's most sexually-aware novel--notice the dramatic, nearly Freudian symbolism of the scene where Maria squeezes around the gate at Sotherton and the scene where Fanny puts the amber cross pendant her brother has given her on a chain. Mrs. Price's excessive child-bearing and Maria's dalliances also suggest sexuality rather directly for a novel written in the 1810s.
Sexuality is newly crucial in this modern world of feigned emotions and performances. Sexuality itself can even be acted out on stage, as the play that the group tries to put on shows. The idea of "acting" is central to the moral calculus of this novel. In a world of mobility, where people move from Bath to London to the country every few months, it is impossible to know anyone's character with any certainty; large periods of their lives have taken place out of your view. Thus, sincerity becomes a crucial quantity. The possibility that someone might be acting--feigning an emotion or even faking their entire character to gain something--is truly threatening when one is making decisions about a marriage partner. Fanny's self-denial and withdrawn nature are not only proper for a young lady, but also an excellent defense. The new pressures of the modern world and the uncertainty they bring lead Fanny and Edmund, the novel's two most vulnerable characters, to adopt a sort of melancholy pose; they seem weary of the world. Melancholy was to become an important concept to the Victorians. Reason, too, assumes critical importance. Being guided by emotions can lead to dreadful mistakes; reason, on the other hand, which often counsels caution or withdrawal, is safe. Fanny, with her keen perceptions and her faith in her ability to reason out how she should behave, is an ideal, if odd, heroine.
Finally, in its resistance to closure--the novel ends with a marriage, but we don't see anything of married life afterwards--Mansfield Park hints at the essential ambiguity of knowledge. Austen cannot give a complete account of Edmund and Fanny's entire life together, so she leaves things hanging. On the other hand, Fanny's marriage has fixed her social position, and she is no longer a single, unpartnered woman, so Mansfield Park has achieved the two major goals of a nineteenth-century novel. In its ambiguity about nearly everything else, however, Austen's novel is revolutionary.
Regarding Maria Bertram and Mrs. Norris in the last chapter, neither leaves England. When Austen writes that "an establishment [is] being formed for them in another country," she does not mean continental Europe. Here, "country" simply means another part of the same country (most likely somewhere in the countryside). They are still in England.