This sounds like a Buckmoth caterpillar sting. You have them in your area. I have them here where I am, as well, and I've had my chickens suffer from their stings. I lost three six-week old chicks when they pecked at them. These caterpillars have nasty spines that have venom, and if your hen pecked at one, she could have gotten the venom in her throat.
You and your vet are doing everything possible to treat this hen for such a thing. The hen may be able to handle one more dose of Benedryl, but more than that would be toxic.
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