Unfortunately, there isn't always a clear cut path when trying to help our friends. A lot is trial, error & frankly praying that you're doing the right thing. Sometimes it all works out & other times there's nothing to be done but apply TLC & a quick, merciful end to the suffering.
It looks like she was riddled with tumors, sadly. There's no way to know that at the beginning of your efforts to nurse her back to health. All of the tumors wreaked havoc on her internal organs & created a lot of fluid build-up, as you know.
You did the absolute best thing for her every step of the way, from recognizing that something wasn't right, to trying to nurse her & keep her comfortable, to ending her suffering & now sharing her story with us.
I suspect that you feel terrible that you didn't know sooner & submit that you were reading the signs as best you could. Instead of feeling guilty that you didn't know sooner, acknowledge that you couldn't have guessed that things were like you ultimately found them to be. While it's a horrible thing & very sad, give yourself a chicken hug from your baby & know that she thanks you for helping her to be at peace & out of pain.
It's the absolute WORST part of having chickens yet it's the most important part.
Sorry for your loss. And take some comfort that this isn't something that many of us have seen before, so there's a rarity factor, too.