Oh my gosh it's beautiful.
Seriously, the combo
Willow Ophelia is
beautiful. I really wouldn't change a thing. If you want to call her
Ophelia, then just go ahead and do it! You won'd be the only parent in the history of the world to call their child by their middle name. And once she's older, she'll just write "
Ophelia" on her papers instead of
Willow, so it'll really be second-nature to her and the people addressing her. There's nothing wrong with using the middle name primarily instead of the first name. Especially when the combo is as stunningly beautiful as
Willow Ophelia.