I really like Aurelia Fern. I like the simplicity, as other posters have said.
Aurelia Pearl (The Scarlet Letter)
Aurelia Iris (flower, Greek goddess of the rainbow)
Aurelia Penelope (faithful wife of Odysseus)
Aurelia Selene (means "moon" in Greek)
Aurelia Niamh (Irish goddess)
Aurelia Aglaia (one of the Three Graces)
Aurelia Clio (Greek muse of history)
Aurelia Lark (nature, Cosette in Les Mis is called "the Lark")
I agree with most everything tori101 said. Of your list, I think Fern or Guinevere would be my choices, with Lyons a distant third.
Aurelia is a name that stops just short of being fussy and frilly, so I don't think you want a middle that would push the sound over that line.
Fern was actually the first thought that came to my head when I saw your post. What about Aurelia Cerise, which I think I love? My other ideas are Aurelia Maris and Aurelia Coraline. Historical and literary suggestions are difficult--who or do you admire; what are your interests? I can see Aurelia Millay if you love poetry or Aurelia Court (after Margaret) if you are a tennis enthusiast; I'm just not sure where to look.
This is a tough one.....most mythological ideas seem too overblown as do most botanical. Most Shakespearean names I've considered seem a little too familiar, because, if I understand you, you want something truly distinctive and uncommon.....Then throw out the names that begin and end in vowels or 'n' and the field really narrows.
Aurelia Leonesse is a lot of name, but I rather like it. It's pretty, flows well, and has family meaning - what could be better?! With such a beautiful a first name like Aurelia, I don't think people will assume you gave your daughter a yooneek middle. Plus, Leonesse doesn't really follow the trendy things people are currently doing to names (this is a good thing!). Aurelia Leonie might be a bit more grounded, if you're interested in that.