okay, going in order:
I agree, [name]Belle[/name] does feel very nicknamey…but I don’t like any of her longer variations (in particular, I loathe [name]Isabella[/name] and [name]Annabelle[/name] has already been used in my family) but it has such sweet connotations that I might just keep it around anyways.
emelis, I love your sibset suggestions (particular [name]Anya[/name] and [name]Luna[/name]) and the suggestion of [name]Persephone[/name] (my favorite goddess!). Not as keen on the middle name suggestions: pretty, but not my style.
alzora, I’ve never thought of an [name]Anya[/name]/onion connection. To my ear, it doesn’t sound close enough to be a bother, so that’s good! Also, I agree that [name]Ariadne[/name] and [name]Demetra[/name] do seem clunky. Not many people like [name]Demetra[/name] either–it holds a special place in my heart, since it belonged to a childhood heroine of mine. Also, they are both unique, mythical and still “heard of”.
and [name]Lori[/name], wouldn’t you know it, but [name]Audrey[/name] [name]Isadora[/name] is on my list already! Great minds… 
Thanks all for the thoughts! By all means though, keep them coming.
Some of my own thoughts for middle names are:
[name]Nora[/name] [name]Madeleine[/name], [name]Winnifred[/name]
[name]Phoebe[/name] [name]Imogen[/name], [name]Cassandra[/name], Willow
[name]Audrey[/name] [name]Isadora[/name]
[name]Luna[/name] [name]Winnifred[/name], [name]Ariadne[/name], [name]Morgan[/name], Willow
[name]Demetra[/name] [name]Lilith[/name], [name]Morgan[/name], Willow