Just to update-
The consensus locally is Ravenseye's #2: I treated too late. Next season the bees get treated according to mite counts and not the calendar. I'm thinking mid-winter treatments while broodless (weather permitting) are also a good idea.
Interestingly, I've lost the hives with the highest (3%) and lowest mite infestation - my two remaining hives are mother/daughter colonies. Time will tell if they make it to Spring.