I am so sorry that you lost the chick. What a horrible thing to happen. We are are so surprised at this strange set of symptoms. If you are able to get your state vet to do a necropsy, let us know what the results were. I would at least speak to the vet pathologist at the state lab just to email...
I am going to tag some veterans on here. @dawg53 , @azygous , @two crows , @Wyorp Rock , @theoldchick and hopefully some others will look at this. Do you have any benadryl? The dosage would be very very small, 2 mg per Kg (2.2 pounds) so I don’t know how little to give.