I think it's really pretty. Very lovely. =] I wouldn't use the NN
Riona... actually, I wouldn't use any NN unless it came from just naturally shortening the child's name. (As opposed to saying, "I'm naming my child X, but will nickname him/her Y.")
If you need more ideas, my current favorite
Katherine alternative is
Katharina. (kat-ah-ree-nah)