The only quest you can still finish in Broken Valley is rescuing the guild master from the quarry tower, and the only one left on Sentinel Island is retrieving the treasure from the boat in the tree (though you may want to use it yourself rather than complete that quest).
People that want to avoid spoilers can stop reading when someone asks a question about something they have not gotten to...
Pulling the other 2 levers would have spawned a chest with some random mediocre loot.
The forth parchment (see link above) could have been translated by ZixZax to give you the password for the teleporters that require one - the teleporter by Lovis' tower brings you to the top of the tower, where there is one chest containing a random piece of equipment, while the one by the chapel shrine gets you to a ledge in the mountain above there with a few goblins and Naberius' storage area, containing a skill book and a little crappy random loot.
The 3 talismans (see links above) could have been used in the derelict tunnel (at the proper place) to give you the Talisman of the North (+2 Wisdom, 3 empty enchantment slots, and a couple other minor random bonuses).
