"Miss Dashwood," cried Willoughby, "you are now using me unkindly. You are endeavouring to disarm me by reason, and to convince me against my will. But it will not do. You shall find me as stubborn as you can be artful. I have three unanswerable reasons for disliking Colonel Brandon: he has threatened me with rain when I wanted it to be fine; he has found fault with the hanging of my curricle, and I cannot persuade him to buy my brown mare. If it will be any satisfaction to you, however, to be told that I believe his character to be in other respects irreproachable, I am ready to confess it. And in return for an acknowledgment, which must give me some pain, you cannot deny me the privilege of disliking him as much as ever."

Is there evidence that Eliza does not respect Col Brandon? If Eliza was the source of Willoughby’s dislike or parroting—as you put it—that would make her a very ungrateful girl as well as gullible and foolish. I think there is plenty of evidence that Willoughby genuinely dislikes Brandon—he says it right out in Chapter 10; IMO this is the most truthful remark Willoughby has made up to this point (Chapter 31) since he swept Marianne up in his Manly arms and carried her down the hill stunning the Dashwood ladies with his Manly Beauty. I do think that Willoughby knew who Eliza’s guardian was, I should think they could not be together without some talking and I do not see how Willoughby could have not heard the rumors about Brandon’s “natural” daughter; I think that his conscious may have increased his natural (for Willoughby) envy of Brandon's wealth into dislike because IMO people often do that--blame the victim for their own shortcomings—it is much easier than trying to do right yourself. (;D)