Invaders/Dahan already in the joined land don't trigger "Ocean in Play," which Drowns Invaders/Dahan after they move into the Ocean, but isn't triggered by them just existing there. When "Weave Together the Fabric of Place" wears off during Time Passes, you can "move" those pieces to the Ocean and Drown them.
Invader, Event, Adversary, Scenario, Fear Card, and Blight Card Actions affect the joined land normally, since it's a land for all purposes, not just for Blight and Powers. Any Invaders/Dahan moved into the joined land are drowned by "Ocean in Play," since the joined land is an Ocean. Note that adding a piece (from, e.g., a Build Action) is not the same as moving it, so Invaders/Dahan added in the land aren't immediately drowned (though they can still be drowned when the joining expires in Time Passes).
If Ocean's Hungry Grasp loses all Presence from that board, the joined land remains in play, since one of its constituent parts is a land. It retains the printed Ocean area as part of its topology and is still an Ocean, so adjacent lands continue to be Coastal. "Ocean in Play" no longer applies to it, so Invaders/Dahan aren't Drowned after moving there, and it's no longer a Wetland for purposes of Powers/Blight (unless it's a Wetland for other reasons). When the joining expires in Time Passes, all of the pieces in the joined land must be "moved" into the in-play component land(s); they can't be "moved" into the out-of-play Ocean.
