You can find the key to the cell on the beach below the bunker (mindreading is not required).
Jack Bolton's corpse is in a wyvern's nest, just east of Camp Freedom. From the talking tree that wanted you to kill some wyverns, fly up the channel for 3 seconds, and you will be at the ledge with the right nest (passing below the ledge with Camp Freedom). If you are at Camp Freedom you can jump of the cliff to the east, follow the only path available, and after a couple drops you should end up near the wyvern's nest, as well.
Arthur Gremory is in the Imp's Lair, just across the water east of the teleporter to Stone's flying fortress.
also need to find grave robbers 1-2-3 (treasure hunters)
There is a door into the harbour building, located beside the elevator platform.