Since my post was going to be long, I condensed the substance of the conversation in ch. 29 to "gossiping does some harm". It does seem to make light of her gossiping, though overall I do think she has a good heart.

Right after Mrs. Jennings says she hope Willoughby won't keep Marianne waiting, she asks when they will be married and Elinor replies
"And have you really, ma'am, talked yourself into a persuasion of my sister's being engaged to Mr. Willoughby? I thought it had been only a joke, but so serious a question seems to imply more; ..."

Part of Mrs. Jennings reply
"I can tell you, for it has been known all over town this ever so long. I tell everybody of it and so does Charlotte."

"Indeed, ma'am," said Elinor very seriously, "you are mistaken. Indeed, you are doing a very unkind thing in spreading the report, and you will find that you have, though you will not believe me now."
It is all over town because she and Charlotte spread the rumors all over town.

You are right, that gossiping can do real harm. Having experienced an untrue rumor being spread about myself as a youngster, I don't like hearing people talk about others behind their backs.