The challenge of an episolary novel - Lady Susan and the film Love & Friendship

Reading for the first time Jane Austen's Lady Susan - not sure why I've never read this one and don't know much about its provenance - was it published posthumously? is it considered unfinished? was it "finished" by another author? is it an early work - or late? - and maybe will read up on this after I'm done, but enjoying the first half (it's very short, a two-night read most likely) and impressed by how well JA works with the epistolary form: what a challenge for the author! She gives up so many valuable tools: the narrative voice, the author's sly asides, the ability to describe and establish a scene (or a character) in the way an author might, which is not the same way a "character' would do so or could do so in a letter (the characrters are not writers, and some of them are not smart, either), the inability to used extended dialogue except is a clunky manner shoe-horned into a letter in a way no letter-writer would do, the need to keep letters short and within reason, in short - telling an entire story through correspondence is tricky (could be and has been done, at least in story format, w/ email and texts) and provides the author with no obvious advantage except verisimilitude: Oh, here's how this novel came to be, someone found a stack of letter! The plot: Lady Susan, the most forward flirt (coquette) in England, comes to stay w/ her brother in law upon her widowhood and plots a marriage for her less-attractive daughter (money) and eventually for herself (freedom). Austen does a great job establishing Lady Susan's character through her own descriptions of her actions (confessional letters to a bestie) and through others' snide and alarming comments about her, esp. the father of a young man falling into her "clutches." Can't remark on this novel without giving huge props to Whit Stillman, who adapted this into Love & Friendship, an incredibly difficult work to adapt and he keeps it in period but in a way even improves on Austen, building out the humor and the sexiness (the suitor who's "a bit of a rattle," or as we would say, has a few screws loose, is just a little goofy in Austen but is hilariously funny in L&F: see his riff on green peas and on the village of Chuchill, which has a church but amazingly no hill!).