This is a slightly out-there suggestion, but have you considered using the initials F.I.A. to get to the nn Fia?
Here are some suggestions (using hyphenated middles if you don't want to change to three):
Other random thoughts:
Good luck! Hope you find something soon :)
Edited for privacy.
Im gonna try again:
Thessaly maybe nn tesla? I like tessa too.
Zahara nn zara?
Amayllis - i love the meaning
Nerissa - maybe nn nessa can come from this
Aiyana - i love the meaning and feel
Gwyneira - a less popular gwen name
I don't know if I've mentioned how much I truly love Willow for you. I may be a bit biased, because it's one of my favorite names, but I think sisters Willow and Eve are fantastic, and she fits well with the other sibs too. I do understand wanting to exhaust all the possibilities though!
Another random suggestion if we're talking about princess who kick butt: Rapunzel. I know she might seem over the top on her own, but next to Pip, Cas, and Z, I think she blends right in. She has a softness and sweetness, but she's also brave and strong and irrepressible. In addition to being a princess, Rapunzel is actually a nature name, referring to wild herbs. Plus the name lends itself well to funky nicknames like the Zelda that you're crushing on :) Just a thought!
Ooh sessha always comes up with good stuff. Rapunzel is cute, and super cool, and princessy without being frilly. And it does lend itself to even cooler nicknames. Plus even though its a little out there, EVERYONE knows how to pronounce it. I think i may have a new name crush. Lol. See what you do sessh? My list is already miles long