I wonder if she may have aspirated fluids at some point if there's some respiratory symptoms. Hens that have Ascites can also sound like they have respiratory symptoms too.
Not saying you caused her to aspirate at all! Chickens don't have a gag reflex, so when they have a fluid filled crop, bend over, etc. the contents can come back up and then they may get some of the food/water, etc. into the trachea which can cause symptoms.
Since she's on an antibiotic prescribed by the vet, I would continue with the kefir or even a little good quality plain yogurt. Antibiotics can make a sour crop worse.
Miconazole, Nystatin or Clotrimazole are commonly used anti-fungal/yeast medications given to treat sour crop. I have used both Clotrimazole and Miconazole so these are something to investigate further. Since you are working with a vet, you can likely get Nystatin.
A molasses or epsom salts flush can be used if you think there's a blockage. I would still work on treating the symptoms, see if you can get the crop to empty with diligence and meds, then if it's still not resolving, try one of the flushes. Make sure she's drinking plenty afterward since flushes can be dehydrating (causes loose stool too so be prepared!). But I would definitely try a flush before surgery is considered.
Did he by chance run a fecal float to see if there were any worms that needed to be addressed?
Hoping the bra is helpful and she's on the road to recovery!