Let's go through your list item by item:
Plot: Lantin, the protagonist, is a government clerk who is married to to a "nice" girl from the country. He lives a happy life with his wife, who develops two passions living in Paris: one for the theatre, and a second for costume jewelry. When his wife suddenly dies of pneumonia, Lantin is devastated and surprised to find that it is difficult to make ends meet on his salary—something his wife found easy to do. Finally, in need of money, Lantin decides to sell some of his wife's costume jewelry, but when he takes it to a jeweler, he finds that all the jewelry is real and worth a fortune! Clearly, his wife had been the mistress of a wealthy man. This unexpected windfall allows him to quit his job and remarry, and although his second wife is very upright, his life with her is miserable.
Setting: Maupassant's story is set in Paris. For such a short story, there is a lot of local detail mentioned. The fancy restaurants where Lantin dines—Café Anglais and Voisin's—are real places, as are all the many streets mentioned in the story and even Lantin's address, 16 Rue des Martyrs. It is fun to look these places up in Google streetview.
Character: Lantin is the major character of the story. He is a minor government official and a bit clueless. He is taken with his own self importance, which blinds him to his wife's infidelity. His wife is a secondary character. She appears in the first part of the story, but her true nature is difficult to discern.
Conflict: The conflict in the story is best understood as an internal conflict, perhaps, or an implied one. In a sense, the true conflict—that Lantin's wife is misleading Lantin and leading a kind of double life—is only hinted at, since Lantin only begins to suspect the truth after her death. In another sense, the story is about conflicts that are deferred or missing. Lantin never confronts his wife, and once he learns the truth about her, he is unable to examine his own motives for marrying her or think about how he could have been so deceived.
Symbol: The jewelry, obviously, is the main symbol in the story. One way to think about symbolic nature of the jewelry is to understand it as a token of love: first, as presents to Lantin's wife from her lover, and second, as a legacy the wife leaves to her husband. Ironically, as long as Lantin thinks the jewels are fake, he can believe in the genuineness of his wife's love. It is when their true nature is revealed that their relationship is proved to be a sham.
Theme: The themes of the story are the nature of love, self deception, and the degree to which one is able to truly know another person. Another theme could be that greed is stronger than love, or that whatever we might believe about love, self interest is always more important.
When reading stories like this, I like to think about what parts of the story might be missing. Here, the whole story of the wife's affair and her lover is missing. It's clear that his wife, for all her country naïveté, was much more savvy than her husband. It may even be that her affair was a way for her to provide for him, since his salary was not enough to support his lifestyle. How interesting it would be to tell the story from the wife's point of view!