Sabina/MGC.
I'm no closer to the exact nature of the guardian itself, but I have determined that Amarinta's group was, in fact, successful in acquiring a sprig of the all-salve. Unfortunately, just one plant wasn't enough to entirely stop the plague that was ravaging the countryside.
Lady Amarinta describes the successful party -- the larger one -- as numbering between two and three score. I'd say somewhere in the 25-30 range, given how many warriors and mages she mentions by name.