BuckSatan, i think the key you're refering to is the key to Jaheira's cell, I dunno if that key will work anywhere else.
But Imoen can pick every lock in the dungeon that's pickable, by pickable, I mean that when you try to pick it, it doesn't give you the message that there's some mechanism on the lock that prohibits lock picking and simple magics to work.
When you get in the jar room with Rielev, look on the table in front of the jar, the stone should be on that table. Then use that stone to activate the Sewer golem.
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