] all the same, he does say that he concluded that Jane's serenity of countenance and air gave him the impartial evidence he needed to support the objections to the Bennet family.

Actually, that's not quite what he said. He said that "I believed it on impartial conviction, as truly as I wished it in reason." However, he also admits that he had reasons for wanting to believe Jane to be indifferent (almost certainly a desire to use her indifference as ammunition; possibly a desire to have his own sister marry Bingley and/or a desire not to be put in a position where he will come into contact with Lizzy). While he claims that his other motives did not influence him, I don't think that he was being honest with himself. I don't see how he could have accept such flimsey evidence if he were not letting his desires influence his perceptions.

] Is he not guilty of trusting his interpretation of people just by one concentrated effort just as Lizzie was guilty of writing him off after the Assembly and interpreting all other interactions in that light?

But there are also differences in the situations. Lizzy had better information, having heard his opinion of her from his own lips. Also, she did not use her imformation to influence the life decisions of two other people. And finally, this can only be speculation, but if Darcy had made an effort to overcome her bad first impression before Wickham stuck in his oar, Lizzy might well have changed her opinion. She was not dead set against him until she believed Wickham.

] Both believed they had a gift of observation and interpretation of character.