It seems as if the Lucas ladies and the Bennet ladies are all talking together. From what I can see, there is no direct dialogue from Lady Lucas, only Charlotte, but there certainly is from Mrs. Bennet, which leads me to believe LL was there too since she's mentioned in the preceding sentence about being such a valuable neighbor. Charlotte mentions "THE SNUB" quite casually so it doesn't seem like it is anything very secret. I don't think it's so much them telling (or not, as the case may be) SW or LL as much as just as Darcy's 10K pounds a year spread quite quickly, so did the news of his snub. I can't find the exact quote, but it is mentioned somewhere during or immediately after the Meryton Assembly that Darcy was popular the first half (before people "knew" him) and then the tide was turned when people became familiar with his manners (or lack thereof). Surely if this disgust was the general consensus, SW would be aware of it, even if there was no participation of the sentiment on SW's part. ;)

That the Miss Lucases and the Miss Bennets should meet to talk over a ball was absolutely necessary; and the morning after the assembly brought the former to Longbourn to hear and to communicate.

``You began the evening well, Charlotte,'' said Mrs. Bennet with civil self-command to Miss Lucas. ``You were Mr. Bingley's first choice.'' ...

``My overhearings were more to the purpose than yours, Eliza,'' said Charlotte. ``Mr. Darcy is not so well worth listening to as his friend, is he? -- Poor Eliza! -- to be only just tolerable.'' ...

As for your statement "I have no doubt Mrs. Bennet would disparage Darcy at any opportunity, but would she do so in a way that insults her daughter, even her least favorite daughter." Mrs. B often does foolish things, such as banding about Bingley marrying Jane at the Netherfield Ball, and refuses to lower her voice even when Elizabeth gives very logical reasons for her to do so.

Not that this necessarily proves my point, however... it was just something that struck me when reading later chapters and wanted to get another viewpoint! Thanks for discussing it! :)