Oh, dear, let me try again:

I THINK you are saying that Darcy does not want Wickham believed because belief in Wickham caused Elizabeth to be prejudiced and condemnatory of Darcy. I think Darcy does not want Wickham believed because belief in Wickham will harm Elizabeth, by leading her astray to be emotionally hurt later when the truth comes out, for instance. My problem with your reading was not the hint that Wickham might have worked his way into her affections, but whether Darcy meant to soften her opinion of himself, or help her in a dangerous interaction with an unprincipled, vicious man.

Darcy gets full credit for being able to change his mind. However, his flexibility on THIS score is helped by an admiration for Elizabeth (the same way a stranger might be given less credence than a spouse, or sibling, even saying the same thing). In Sense and Sensibility, Elinor points out that another character's good principles are not only proved by the actions he takes to help a loved one (which even an unprincipled person would do), but on many other references, and actions. I am saying something similar here: Elizabeth changing her mind at the instigation of a man whom she personally, thoroughly dislikes is harder than Darcy's changing his mind when the woman he admires and recently wanted to be his bride differs from him - and in a point where she is almost certainly better informed than himself, Jane's heart.

Elizabeth DID believe Bingley, by the way: she believed he had been told by Darcy that Wickham deserved to lose Darcy's regard, and she believed that Bingley believed it, himself. She simply did not believe DARCY.