there was an understanding (i.e., engagement) between the two that had not yet been announced. The only way it makes sense to me that Mrs. Gardiner would be astonished at Lizzy's ignorance of Darcy's role, is if she thought Darcy and Lizzy were in communication about it. That does not necessarily mean that Darcy consulted Lizzy or that they planned things together, but that he informed her by letter of all that he was doing to find Lydia and forward the match. This was a very delicate affair, but it seems to me that the Gardiners were considering Darcy as (almost) one of the family. I don't think they would have let him do as he wanted, had they known that Lizzy had once rejected Darcy's proposal and he had not yet proposed a second time.