I don't like the look of Evvy either. You're right Evy looks better. I just don't want to put Evanna on her birth certificate because I'd never ever use it anyway. I would give her a feminine middle name, though.
Evanne is kind of pretty but I'd want to say ee-VANN (kind of like Eve+Anne). Maybe for a character. I know a
Ryanne too. She also pronounces it like
Ryan but before I met her (she was in my class and I got the class list before meeting everyone) I thought it was ry-ANN.