The fact that Elizabeth waited a month before deciding she could never marry Darcy is evidence against the idea that she lost all objectivity about him after his insult at the Meryton Assembly. (Though I think that he was on her list of candidates only in the sense that eligible men were few and far between, and marriage was a financial necessity, not that she was personally attracted to him.)

Thanks for pointing out that "I had not known you a month" coincided pretty closely with her hearing Wickham's story - I never noticed that before! I think it's very significant!

And yes, if Darcy had been consistently polite and reasonably friendly (just the basics, nothing out of the ordinary), I think she might have accepted Wickham's story less quickly, though I don't know if it would have been enough to prevent her from siding with Wickham eventually, without more evidence.