I think gentlemen were given the benefit of the doubt, they are believed to be gentleman and moral individuals until they do something which changes that opinion. To use your example, if Darcy had danced, been amiable, not despised people in general but hid his immorality well then who would know? If no one knew then people would still think he is a gentleman, right? Darcy was expected to act in a gentlemanlike manner until he did not. Society then rightly questioned his behavior and attributed it to the correct reason: he did not feel incumbent to be agreeable because he believed they were unworthy of the attention.

Darcy behaves in a way that makes folks think badly of him. When Lizzy hears he has behaved even worst towards Wickham it is not so far a jump to believe his story because Darcy has treated folks badly before. Instead of imagining Darcy behaved correctly at the assembly why not look at the Wickham story substituting a gentleman who did not create a bad reputation for himself. If Wickham had accused Bingley Lizzy would not so easily have believed his story because she has never witnessed Bingley treat anyone badly. In Chapter 16 when Wickham tells Lizzy his story Darcy’s reputation precedes him and makes a significant difference in how Lizzy judges his capability to mistreat others.

"Some time or other he will be -- but it shall not be by me. Till I can forget his father, I can never defy or expose him."

Elizabeth honoured him for such feelings, and thought him handsomer than ever as he expressed them. (Chapter 17)

We do not interpret Lizzy’s comments the same. Lizzy honored Wickham for his respect for Darcy’s father and thought him handsomer for having such selfless feelings. I do not see anything wrong in Lizzy admiring this sentiment. Wickham’s open countenance does vouch for his being amiable, does it not? Lizzy finds Wickham amiable and she likes his open manners; do you have a reason she should not enjoy his company or like him at this point of the story? To me, there was truth in his looks means Wickham’s open and gentlemanlike manner enhances the honesty of his confession. She did not see any signs in his manner that he is being deceptive. I do not see “truth in his looks” as meaning: He is handsome therefore he is honest. Yes, I think Lizzy means Wickham appeared to be sincere. ;D

"A thorough, determined dislike of me -- a dislike which I cannot but attribute in some measure to jealousy. Had the late Mr. Darcy liked me less, his son might have borne with me better; but his father's uncommon attachment to me irritated him, I believe, very early in life. He had not a temper to bear the sort of competition in which we stood -- the sort of preference which was often given me." (Chapter 16)

Darcy is proud and disagreeable at times but I have not tried to make a case that Darcy’s pride led him to withhold the living from Wickham. Wickham does not say Darcy withheld the living because of his pride. Wickham attributes it to dislike and jealousy which is believable to me considering Darcy treated Lizzy very badly only because he looked down on her socially—not nearly as bad an offense as stealing your father’s affection. What might not an ill-temper disagreeable man do to someone he both dislikes and envies.

Yes, there are other things that make Darcy a gentleman if you want to make that case but how does that excuse his behavior at the assembly? What other behaviors makes you think his bad behavior should be overlooked when Lizzy hears Wickham’s story? If you feel Darcy’s behavior was not so bad then can you say why? ;D