I had never thought of connecting the two incidents as you have, and the superimposition is very interesting. I haven't yet made up my mind whether I agree with you. Here's what argues against it.

Did generations keep secrets from each other then as they do now? Would Charlotte (or even Maria) have told of Lizzy's overhearing to her parents? I'm not sure Charlotte would. I am less sure of Maria (unless cautioned by Charlotte). If Sir William heard, it would far more likely be someone like Lydia, but she might not talk much to Sir William.

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. Bennet has her own version of pride and IMO would never want Lady Lucas to hear of Darcy's disparaging comment about a member of her family.