] My favourite conspiracy theory is that Charlotte somehow told Darcy in some manner that Lizzie would be visiting Rosings at a certain time. After all she had her suspicians and the visit was planned a long time in advance when Charlotte and Darcy were in the same social circle.

Mr Collins did not propose (to either Elizabeth or Charlotte) until after the Netherfield Ball. By the time he proposed to Charlotte, Darcy was back in London -- so Charlotte & Darcy were no longer in the same social circle. And none of the Netherfield party returned to Netherfield prior to Elizabeth's visit to Hunsford.

Do you really think that Charlotte would write a letter to Darcy in London to tell him that Elizabeth would be visiting in the Spring? This seems highly doubtful to me. Even if Charlotte thought Darcy was interested in Elizabeth, it would be quite presumptuous of her to get in touch with Darcy by letter, and I don't know of any acquaintance that they have in common who could serve as a messenger or go-between.