They started stacking stones when they got stuck on the island, and she symbolically stopped with the seashell when they were free. It is the intended final addition. Kinda makes it an informative monument, they have a sort of physical record of how many generations were stuck on the island.
Yeah, she leads them off the island and they become voyagers again. Her placing the shell from the sea was to imply they have returned to the sea.
Well when the villagers inevitably forced to return to the island, I imagine the next rock to go up will effectively crush the shell. Just like crushing her legacy.
