I'm sorry you think
Rowena needs a nn.
Wilhelmina is very long and comes with a truckload of possible nns, none of which I like on a girl.
Edwina is beautiful, elegant, dignified and stylish.
Eddie, not so much!
For me, it would be a toss-up between
Rowena and
Edwina: I'd want to use
Edwina for family reasons, but I'd rather have it as a mn except it doesn't go well with
Rowena!
Since two out of your three are fem versions, might you consider other fem versions as well? I'm thinking
Antonia,
Cecily,
Harriet ...