I'm sorry about your hen.
A way to find out more would be to have a necropsy performed. Some send the body to their state lab while others like me do an informal one to see if there's anything obvious.
Depending on where you live state lab services can be reasonable, some are not.
You can look...
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From the sound of it, she's had issues most of her life and her condition is likely worsening over time. Often hens with Salpingitis also accumulate lash material in the abdomen as well.
I agree, I would try offer a little wet feed, bits of egg to entice her, etc.
Hopefully...