Daniel Cassidy's "How The Irish Invented Slang" (CounterPunch & AK Press, 2007) has an entire chapter dedicated to the etymology of "Jazz" including reprints of

Daniel Cassidy's "How The Irish Invented Slang" (CounterPunch & AK Press, 2007) has an entire chapter dedicated to the etymology of "Jazz" including reprints of articles mentioned in the OP. It includes much of what Porter writes about, but is a much deeper dive. It's well researched and worth the read, but with a grain of salt. Where Porter's parenthetical (I think “gin-i-ker” means “full of gin.”) is a shot in the dark, Cassidy's polyglot ear for slang catches the Irish "tine caor" (pron. jin-i-kær) meaning "raging fire and lightning," delivering a livelier and more nuanced read. You'll see mention of Scoop Gleeson, but find out he's holed up 40 miles north of SF in Boyes Hot Springs, CA covering the Seals spring training. There may never be a definitive answer to the headline question, but as far as I know, this is likely as close as you'll get.