I finished the novel at last!
The novel is more leisurely, and thus the assumed cause-and-effect the movie pushes on you really doesn't exist. Marianne's illness is not the direct result of hearing of Willoughby's betrayal; there are weeks -- maybe over a month! -- between the two. They linger in London for weeks as Mrs. Palmer recovers from childbirth and Mrs. Jennings attends her.
The most boring part of the book is in this section when Elinor must visit with her brother John, who is Boredom Personified. Fanny's family is not much better.
Lucy does not immediately throw Edward under the bus in favor of his brother Robert. She keeps Edward on a string, writes to him, avows her faithful love for him .. and suddenly runs off with his brother. She didn't have designs on Robert immediately for his inheritance. Robert visited her to convince her to free Edward of the engagement, his familial duty. She required repeated visits, of course, and gradually the purpose of the visits shifted. Robert did not receive all the family money; apparently Fanny ended up with quite a bit. And Mrs. Ferrars gave in later and handed over a bit to Edward, allowing him a more comfortable married life. Austen has much fun at Mrs. Ferrars's expense, ridiculing the fickle matriarch with her inconsistent punishments and obvious favoritism.
Perhaps the single most astounding lapse in the movie is the omission of Willoughby's visit to the Palmers' house to see Marianne on her death bed. He does run on and on for pages, attempting to convince Elinor (who never allows him to see Marianne) that he's not such a scumbag after all. He admits to having a true, loving attachment to Marianne at one time, which is about the best he can say for himself. Even tender-hearted Elinor sees through this, however. She knows the man chose his need for cash over his love of Marianne, which means he didn't love her in the first place. Austen makes it clear that Willoughby didn't need Mrs. Grey's 50,000 pounds a year for necessities, but for his extravagant lifestyle -- his personal addiction. Of course, knowing his treatment of Eliza and his illegitimate child rather stripped any gloss from all his avowals of love. A slug he was, and a slug he will remain.
Elinor marries Edward as the movie depicts it (which is nice), but it was not a double wedding. The newlyweds moved to Delaford to the vicarage where Colonel Brandon oversaw the refurbishments of the house, and Mrs. Dashwood and Marianne visited regularly. Brandon is over 20 years Marianne's senior, and the romance is only barely such. He adores her, but she is won over at last, according to Austen, simply because she could not withstand the desires of everyone she loved. She had resigned herself to a single life based on her Love Maxim. It seems to me that Emma Thompson, in writing the screenplay, kept her own character's romance quite close to the book but allowed her character's sister's romance to stray considerably.
Austen makes the point quite plainly: Marianne must confess that all the best loves in her own sphere were second loves. First loves are often foolish (like Edward's) or disappointed (like her own), or prevented (like Brandon's). Second loves, undertaken more wisely, are more likely to succeed. Thus, sense conquers sensibility. Elinor's self-controlled head-over-heart methods prove more admirable than Marianne's catapulting-over-the-cliff with one's heart. However ... however ... Edward is Elinor's first love, yes? And her love for him is the strongest and most faithful. Perhaps Austen's message is not that second loves are better, but that first loves for which one waits cautiously and patiently are best of all.