If you have a big box pet store like Pet-Smart they will have a product in the fish supplies called Fish-Zole, it will come in a packet form you mix in water and tube to the bird. Also look for a crop needle or Gavage feeding syringe while you are there.
I expect that KKs hen has blackhead or a bad yeast infection. I am not sure the vet can run a good fecal exam. BH will not show up on a fecal exam but the worms that carry BH do. I have seen that black tarry stools in both BH and yeast infections. These are pics from my first experience with...
Here is a photo of E-Coli in young chicks, I had a really bad bout with this and still have a few recovering. They are very slow to recover. Their poo is a slow drizzle of whitish yellow.
Oh, that is not good at all. I would start her on metronidazole right away. 125 mg 2x per day or 250 mg tablet once per day for five days for treating blackhead. When done give her a probiotic to rebuild her gut flora. If you can't get to a vet for the metronidazole today you can find it in...
I am not impressed with your vet. If the fecal exam was clear of cocci and worms why treat for cocci? It would take a month or more for a stressed bird to have symptoms caused by cocci.
It does sound like a bacterial infection since the poo is white to yellowish in color instead of the...