Your train of thought reminds me of the debate about whether Marianne is a full co-heroine or a more secondary character. My current thinking is that she's secondary in the novel, but on screen her significance catches up with Elinor's, because the reader/viewer no longer sits in a privileged position inside Elinor's head. Perhaps, in the epistolary version, the important conversations between Lucy and Elinor were still hidden from the reader at this point in the story?