Kind of a silly question, but is Nala a social eater? Would she be more adventurous with foods if you tried them with her? Or even prepared them with her on your hand or shoulder (if it's not dangerous.)
Also, maybe there's a fruit or veggie she won't usually like that she'll eat prepared another way. For example, Afie will only chew up raw carrots, but she eats cooked ones very carefully. Also, there's some foods she doesn't love, but will eat if she has to forage for it, because it's like a little treat. There's a lot of nice food toys as well, like an artichoke hung upside down in her cage with healthy food stuffed in between the leave-things, or even just a simply kabob.
Sparky and Junior are both very picky eaters, but there's a vegetable mix we make and freeze that they love to eat every day. Also, they'll be more likely to try something if it's in Afie's cage. I don't know what Nala's relationship with Simba is like, but maybe she'd eat in his cage?
I know you've tried a lot, but there's bound to be something out there she likes. I've heard that GCs like bell peppers, and all of our birds LOVE soybeans.
Best of luck!
P.S. You are a great parront!