One of the letters that would have moved the plot forward, had the original novel been an epistolary one, appears in Ch. 31. Elinor is writing her mother, describing the denouement of Willoughby's behavior.

(By the way, I have tried to imagine writing this letter, and it is painful for me to even contemplate composing something to send to Mrs. Dashwood, fictional though she be, that would distress and shock her so much. It must have been five times worse for her daughter to do it! Poor Elinor! Poor Mrs. Dashwood in reading it!)

However, the letter-writing is interrupted by Col. Brandon's entrance. By rights, had this been a novel of documents, the tale he tells would have been written, too.

However, in addition to the timing changing, (for Elinor would hardly interrupt herself writing to her mother, to read his letter), the wording of Col. Brandon telling his backstory would have been very different. Had he realized half through that he needed to be more collected, he would have rewritten it prior to sending it, not simply pointed out that he was a bad narrator, and would finish in a more concise style.

Also, while he certainly has the floor, Elinor's reactions are important to the feel of what he relates. He is pleased that she remembers his mentioning Marianne's resemblance to Eliza; he sees Elinor's reaction to the distress he relates, which forces him to combat his own discomposure; he stops to reassure her that Marianne's fate will not reflect that of Eliza; and then the mention of the duel, which I do not believe he would have told her, had she not asked her questions at the end.