I don't think lack of documentary evidence can be scimmed over for a potential marriage of this magnitude. Lady C talks of 'the marriage being organized from the cradle'. I find it hard to simply take Lady Catherine's word alone without at least some correspondence between the sisters or between their lawyers. In the Eighteenth and Nineteenth Centuries, marriages of the aristocracy were often planned for years in advance by parents, usually with attorneys in relation to whether the wife kept control of her own property share, childrens' marriage portions,etc. If Lady C. had a letter at least mentioning the possiblity, I think she'd show it to Lizzy when she confronted her at Longbourn to try and extract Lizzy's promise never to marry her nephew. For speculation, if Lady Anne shared her sister's attitudes towards class and and wealth, it doesn't neccesarily follow she will enthususe about her niece as an 'excellent match'. It is a typical attitude of the aristocracy to keep all options open. And it would be more to the advantage of the Darcys than the de Bourghs to do this. With their estate and income, the Darcys are apparently one of the top families in the country. As well as his heiress cousin, Darcy's parents could be thinking about encouraging him with the daughter of a duke or an earl. Even if her fortune didn't quite match Anne de Bourgh, Darcy has the great income of 10,000 pds.Darcy doesn't necessarily need a great heirss. If she was typical of class attitudes like her sister, Lady A may prefer Darcy to marry a titled lady. There is also a disadvantage concerning her niece. Namely, Anne is sickly and unwell. At this time, when selecting marriage partners the upper classes took health into account. The first duty of the wife was to produce a living heir. If Lady A was arranging her son's marriage she may not dismiss the idea of her son's marriage to Anne but I think if she did, she'd keep options open. It was really more in the de Bourghs interest to secure this match. Once again, this is all speculation. As you say, Lady A is an enigma and we don't really know enough to make firm judgements of her.