As a last installment to Paris' story, I am including pictures of her necropsy. If they can help anyone else understand what might be happening with their own bird, I offer them. I spoke with the vet, and she said she felt it was probably an oviduct tumor. Paris' oviduct was filled with fat from ovary to cloaca. Her ovary had very few developing follicles, and no mature follicles. Her liver had one tiny white spot on the margin, but was otherwise unremarkable. The vet said her liver would have been very abnormal if it had been the cause of the ascites. That is ruled out.
First pic is of her oviduct.
Next pic is of her ovary.
Last pic is of her liver.
Was she an internal layer? I don't know.
Did she have an infection? Yes.
Was this preventable? I don't know, but probably not.
If you are looking for answers for your own bird, I wish you the best!