Nevena is also a Croatian name and Neven is a male word name here (the noun marigold - neven is male here). I like Nevena and Nevenka, but it's considered dated here. We use Nena as a nickname, but I also like Nev (spelled this way).
Nena is so sweet! I guess now, after hearing your opinions, I would spell it "Nev" rather than "Neamh". Nev is more pratical, and makes more sense as a nickname for Nevena.