a seperating zipper very well could work in a purse of the right design.
it isn't fair to expect folks who work in a fabric department to be experts at all types of sewing. do quilt shop employees all know everything there is to know about all types of sewing? i doubt it.
you're being awfully rough on that poor employee. you sent your daughter for a 12" zipper. it's hardly the employee's fault she settled on a 22" instead.