I'm sorry she's not improving.
If you do decide to end her suffering, you may want to consider sending the body to your state lab to find out more. https://www.metzerfarms.com/PoultryLabs.cfm
How much Corid are you giving?
By any chance she's not gotten into anything toxic - oil spilled, antifreeze, something moldy, fertilizer, etc.
The desperately thirsty is very concerning - is she drinking all the time?
Has the breathing straightened out or is it still crackly?
Are the pellets an all flock or layer feed?
The crop going down some is good. Even if she's not eating, if she will drink to keep hydrated that would be promising.
Oily poop? Or maybe mucous?
Looking forward to the poop photos (that sounds odd doesn't it)
Crop issues can be from a number of...