Personally I think we make way too much tado about cores. They are pretty cheap and mine have worked for three years with simple freshwater flushes. IMO, don't over think it. It's the pre-filters you need to maintain more than anything.
Our course is set for an uncharted sea
Membranes have to be kept moist at all times. Usually they will keep for a year or so in the packaging they come in. They should be stored like this in a cool place. But once it is installed into a watermaker then it needs to be kept flooded with either a pickling/storage compound or flushed on a regular basis if not in use. Don't poo poo the cost of a membrane if it has to be shipped. Many times as with all marineparts the shipping and duty can exceed the cost of the part.