The correct lever in black cove is in the same room as where the skull is (on the island there is a lever, but only to bring you back to the black cove start).
The lever is at the bottom right, move down the stairs there and then take a look to your left at a bush, to the left of that bush is where the lever's at.
Black Cove & White Witch are not linked to eachother, but the white witch lair can be found in Luculla at a house protected by a magical bubble (southern part of the map). You'll have to get inside in order to get to where the witch is at.
