I think
Greta's fine on its own. it doesn't sound especially nicknamey to me. I don't personally like it a great deal, but it is cute.
I like
Ingrid and I would
love to see it on a little girl. I find it adorable.
Evadne (which I love) sounds good with both names, imo. The styles may not really match, but I still think they're nice together.