I finally got around to reading this. Gorgeous, and so moving. I had to go back and re-read it all when I got to the end, in order to try to understand how Elizabeth could have been alive all along. You were very subtle and clever about it--especially the "letters Elizabeth would never write again." I would have been okay with her dying, because as you say, death will come eventually anyway, even as it has come to them in the form of their children. Yet I am glad to think of them continuing on together.
I do have one question--the image Darcy thinks of, Elizabeth dancing with three children. Is this supposed to represent the three that died, or three living children they have now? I am mother to three never-born children of my own, so that particular part of the story particularly spoke to me. I never had to suffer through a still birth, thankfully.