Not sure I ever saw the word "chantwell" before 1970 or so. I had always assumed it was relatively modern Euro-American slang (re)adopted-adapted by islanders; the usual French to English manglish and folklorization. What is the earliest usage in the sources?

In my experience, "chantwell" was only for the French Christians of Trinidad. The English speaking Spiritual Baptists, descendant of the Merikins and the Gullah, had "First Shouters" (no longer accepted usage.)

It's all, mostly, based in Christianity. They're all the head music person in the company - historian, teacher, lead singer &c.