Alcohol stoves work fine on the JMT. For two hikers on the JMT in August we averaged three ounces of fuel per day, cooking twice a day and not conserving fuel at all.
I was at Rock Creek last weekend (Feb. 18-20, 9,500’, east of the JMT near Mono Pass) and alcohol stoves worked fine. Temperatures ranged from 18 degrees to minus 8 degrees. Stoves take more fuel when it is cold and it is helpful to keep the fuel bottle inside your jacket.
Robert