I agree with what you say about the Juvenilia and the History of England. However, on the origin of Mr Darcy's name, just one or two points:

] As to whether she implied anything to do with either the Whigs or Catholicism in the use of the name it's even more vague to me. One thing I'm sure of though, is that she couldn't really have applied both.

I'm far from sure about that. By the time P&P was published, the Whigs were pushing for Catholic Emancipation, while the Tories, even those who were quietly in favour of it, like Castlereagh, abided by the King's will and opposed it. (This was something of a reversal of the position that first formed Whig and Tory, since the Whigs had been the ones who supported the Hanoverian Protestants, while the Tories were those who hankered after the Stuarts).

However, I don't think that JA was indicating support for Catholics and Catholic Emancipation, since there was another Darcy whom she might well have heard of, who came from the Irish Protestant ascendancy like Tom Lefroy. This was D'Arcy Wentworth, famous highwayman. For more on him, follow this link.