Final update.
Miss Echo had decided that the only foods worthy of excitement were sprouts and watermelon...hard to gain weight on that diet.
We had a couple mild days so I put her out in the small quarantine coop with a friend, still no interest in eating actual food.
It was a difficult decision to cull as she was still very perky, hopping on and off her roost, lots of talking, and came running for sprouts and watermelon.
After culling I decided to do a home necropsy. I regret not taking pictures as everything was easily visualized since she had no adipose tissue. Anyway, I'm glad I did it.
The mid section of her oviduct was black and necrotic, it basically fell apart in my hands. Inside that necrotic tissue was a piece of soft egg shell. I'll never know if she stopped laying because of the infection, or if she stopped laying when she started to molt and this was an unfortunate hiccup as her reproductive system slowed down.
Otherwise her organs all looked healthy, no evidence of tumours, etc. The incision from her crop surgery had healed beautifully.
As far as treatment, I did everything possible. Short of putting her on antibiotics back when she stopped laying, but at the time there was no indication. And I'm glad that I made the decision to cull before she got septic and really felt awful.
It's possible that she hasn't been feeling well for awhile and decided two months ago that the only foods worth eating were grass/sprouts/watermelon, resulting in her gorging herself back in November and ending up with the impaction. She has always had access to grit, but I didn't find any in her gizzard. Hopefully my other girls are consuming it and not just throwing it around.