Possession isn't possible because they aren't entirely strong enough spiritually, but communication IS possible! Those who can connect with spirits will find it easier to communicate with them, barring the low-frequency setting stipulations.

They don't particularly have any stories to tell on a game-wide level, though an individual spirit can of course have their OWN story to tell. They are indeed part of the setting, not just random ghosties. It can be assumed they're spirits of previous inhabitants, perhaps of the world as a whole and not just the island chunk itself.