There may be something in the intestines or other parts of the digestive tract that the sulfa isn't able to respond to, a blockage such as a tumor. There's no way to know, but an infection should be responding to the sulfa by now, and the crop should be draining better than it is.
I'm only bringing this up because it would explain why we aren't seeing the progress we should be seeing at this point. It doesn't mean that this is a diagnosis. Even a vet wouldn't be able to tell without fancy imaging equipment.
All we can do is keep up with the meds, and hope they kick in soon.
One thing I've tried when a crop refuses to empty is to massage very gently, keeping to the lowest part of the crop, while cupping the crop in the other hand to elevate it so it is even with the crop drain on that diagram. This will help empty the crop unless there's a solid blockage downstream preventing it.
Yes, I'm very worried, too. Sometimes doing all we can isn't enough.