I don't recall having to do anything special to get the sewer open, but had the relevant quests before I got there.
Did you talk to Bernard, in the Healers' House in the Great Market plaza, about the food thefts?
Did you talk to Darvesh, the beggar by the Mardaneus Plaza shrine, and ask him about the missing healers?
You don't need to mindread Dwayne to access the storage room in the sewer, that just tells you what you need to look for (levers on some of the columns on the path from the entrance). From Lancet's
DKS FAQ/Guide:
In the Sewers of the Crow's Nest. The northeastern room contains
treasure, and this is in the chest hidden in the back of the room. To open the
gate, three secret levers must be pressed. From the entrance:
1: One on the second column to the left.
2: One on the fourth column to the left.
3: One on the fourth column to the right.
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