Klag Orrack:
I'll go check on Trace and the boat docked out back he's around here some place, and we'll find him even if I have to burn the whole building to the ground.
Klag will move down stairs, and find Trace before going out back to the boat.
Klag, getting 'downstairs' last time required a jump in the water. You can do that again of course. Or you could try to go back down the way Guillemot went up through the trap door but it's a DC 20 escape artist check to get through. Or you could go back to the walkway on the South side of the building and try to climb back down to the lower walkway, though there'd be a risk of falling in the water.
Let me know which route you want to try. Otherwise I'm going to assume you jump in the water at G10-H10 and swim to the lower walkway.