"choir" and "chorus" originally meant the same thing - a chorus line from the Greek band of singers and dancers - "choros" and the IE "ghoros" - an enclosed space. Not that I'm putting that forward as a serious derivation. The spelling of choir in English only settled down in the 1850s and it is still pronounced as "quire" and not choyre. How is the Welsh "Cor y Cewri" pronounced? It is the sound not the spelling that signifies and often misheard or misunderstood names were incorrectly written down and adopted.