] I'm puzzled by Mrs Gardiner's recollection at the end of Ch.25 of Fitzwilliam Darcy as 'a proud, ill-natured boy' Does this reinforce (first time) readers notion Lizzy's impression of Darcy as correct?

Not necessarily, because there's plenty of hedging in the passage that could lead a reader to believe Mrs. Gardiner's memory on this subject is faulty. What the passage says is that after Wickham gives her his long, sad story about Darcy's treatment, Mrs. Gardiner tries to remember something about Darcy as a child that would be consistent with Wickham's account of him and finally convinces herself that she remembers she'd heard him spoken of as a proud, ill-natured boy.