Daud has left Drapers Ward on Lizzy Stride' s boat and could arrive within hours. Disable the grave switch. Isabelle will lock the manor and wait for further instructions in the back yard.
And just think of this, what do you think would happen if Duad would have just rushed through the front door?, thats right witches would most likely attack and we all know that eventhough Daud is turning over a new league he would still want to get the job done by any means. So mabey thats why she hinted about the lever instead of giveing him the key.