that both the letter and the house worked on Elizabeth's change of feelings, and as to the chicken and egg issue, as the letter came first, chronologically, it would seem that the letter did the trick first!
I like your development about the impact of the house on Elizabeth. I find it quite enlightening, and it makes a lot of sense to replace it in the context of the period.
As to the letter, when reading Lila's post I came to wonder about another possible reason why Lizzy was turned around : she realised that Darcy had suffered through W.'s villainy. We know Elizabeth to be a good person at heart, even though she does not see the world in so benevolent a light as Jane. Compassion for Darcy's sufferings? Could that be an ingredient in her transformation? Or is it sheer fantasy on my part?