] I think most readers at this point don't really like Caroline Bingley very much, so when she warns Lizzy about Wickham, the reader dismisses her statements as readily as Lizzy does.

This isn't my first reading of P&P, but I hope the first time, I dismissed the information not because I didn't like Caroline but for the same reason Lizzy did -- because all Caroline is doing is revealing her own prejudice. Lizzy herself puts her finger on this when she says, "His guilt and his descent appear by your account to be the same ... for I have heard you accuse him of nothing worse than of being the son of Mr. Darcy's steward." Caroline didn't disbelieve Wickham because she had inside information. She didn't disbelieve him because Darcy has a good character (if indeed he does -- we don't know yet) or because Wickham has a bad character (if he does). She disblieved him because he was the son of a steward and (though she doesn't say this in so many words) Darcy is the grandson of an earl. Pretty flimsey evidence.