It never occurred to me that Caroline doesn't think she'll fail in getting Darcy, but she does seem very jealous and that jealousy makes her act out in the only manner acceptable at the time (biting comments). But your point is well taken, and I like the Caroline/Col. Fitzwilliam idea but her nasty attitude as a mask for her insecureties leads me to believe spinsterhood or a loveless marriage not unlike her sister.