to your question, Emmy : "how would Darcy have explained to CF that EB may ask him some questions"?
Without saying anything against the truth, Darcy might have warned Col. F. that Elizabeth had formed an acquaintance with Wickham in Hertfordshire, and that the latter had spread all sorts of falsehood about him and his family. This could include fallacious details about his involvement with Georgiana. In order to set things right, and to defend his sister's reputation and honour, Darcy might have had to reveal to Elizabeth, as a friend, the particulars of the story.
Would this seem plausible to his cousin?
Or is all this too far-fetched?