Thanks for the detailed answer!
I experimented with containers, and they're not what I was expecting but they're really useful.
However I don't think I can use them because the way they work, from what I can tell, is that one of every object in the container is spawned at once, and each one is linked solely to the objects it spawned with. This means I can't have more than one of the same object, e.g. more than one eye, and still have them linked together in the event sheet.
I could bypass this problem if there were some way to spawn a container full of objects, then spawn another instance of one of the objects without anything else in the container getting duplicated too, then add this extra object into the existing container. But I have no clue how that would work.
Any ideas, anybody?