I have been enjoying this story, but find the title business a bit disappointing.
I understand what you were getting at with regard to the last names, but I think hyphenating the last name would have been a better solution. I can only imagine that the elder Darcy son will be
very resentful for having his intended status taken away from him (at school, he will have to give way to titled boys he should have outranked.) And as Maria V has mentioned, throwing away the title also throws away opportunities for good works--seems like a high price to pay for Darcy's pride.
BTW, have you ever read of peers giving up their titles in British regency history? Seems more like an American democratic fantasy (I am an American).
Also it seems to have been a waste of time for James to begin the settlement business before the name business had been decided. Surely, with D&E deciding to be Darcys, E will no longer be the heir to most of James's holdings.