Elinor's number one priority was to not let Lucy know that she cared for Edward, to not appear jealous or overly interested, or to acknowledge being rivals and engage in petty fighting. That was the purpose of the talk over the filigree. To not promise to keep Lucy's secret, Elinor would be stopping to the petty fighting she wanted to avoid

As for the SUTH, yes, I think this is more the kind of SUTH you give on the way to scratching someone's eyes out. I can understand exactly what Lucy's motivations are for telling Elinor, but she went completely over the top. Telling Elinor that she and Edward were engaged and concerned about his mother's disapproval should have been enough, without having to tell a sob story, and intimate details of her relationship, including how much they loved each other, and constantly rub it in Elinor's face that Edward was spoken for.