Is she acting any better since you drained the fluid?
She may have something reproductive going on - Cancer, Tumors, etc. and/or organ failure, hard to know.
I'd try the Amoxicillin to see if that makes a difference, but to me, if she continues to decline, then it may be time to put her out of her misery. If you do, then since your Mom is wanting to process her, take photos of her organs, the abdominal cavity, reproductive system, etc. You may find some answers.
Here's your other thread, yes, at the time, it seemed she would benefit from extra Calcium. It seems she's much worse now than she was at the end of August.
https://www.backyardchickens.com/threads/is-she-egg-bound.1596563/