That's very sad that your new chicken became sick so soon after getting her. Unfortunately, we would probably offer advice similar to what you're already doing. The symptoms could be from any number of diseases, and sometimes, we can only try different treatments until, hopefully, something works.
Your decision to try Corid is a good one. Be sure to do the full five-day round. In addition to mixing it into the drinking water, you may also give her about a quarter ml liquid Corid undiluted for the first couple days to give her a good launch into the treatment. I hope it works. Wait a week, and treat for five more days.
There is also a possibility that she has Marek's. She's right at the age when symptoms first show up in a young infected chicken.