IMO Lizzy probes Darcy's character because, of those in her acquaintance, Darcy is the only one whose character is under dispute. She knows Bingley holds Darcy in regard and Wickham has accused him of dishonorable conduct. I see no puzzle--indeed a little magnanimity--that she gives Darcy a chance to explain himself, even if she is already partial to Wickham. Until his post-proposal letter, however, Darcy does not give Lizzy the specifics she needs to decide. (He warns her--does he not--not to try to sketch his character based solely on what she knows. :-)

The only other time a character's character is in dispute for Lizzy is when Charlotte accepts Mr. Collins and Lizzy's faith in her is shaken. Then, too, Lizzy tries to probe Charlotte's reasoning and re-evaluates the relationship (and continues to do so even much later at Rosings).

So Lizzy's interest need not imply an early romantic interest in Darcy. Any interest Lizzy had in Darcy was related to her own pride, that pride--or more properly vanity, according to Mary's definition and Lizzy's later admission in Ch. 36--he wounded at the assembly. She was IMO determined that he would accept her as someone of account, and so she challenged him intellectually at the Lucas party, during the Netherfield visit, at the Netherfield ball, and at Rosings and Hunsford. She didn't like him; she didn't care whether he liked her; but she managed to fluster him as much as he did her, and I think she took some satisfaction in that until the first proposal (not to mention the letter) put a new spin on everything that had passed as well as anything that might happen in the future. In light of Darcy's declaration of feelings and Lizzy's response, each had to re-evaluate the effect their words and actions had on the other. In short, each had to empathize more carefully in order to judge the effect their words (not to mention their wit) might have.