For the eastern centre cell, there may be rats in the cell before you get the quest to convince Edmond he is a ghost, but the one you need to kill (and a couple of his friends) only show up after you have the quest. Return to the cell and move towards the back wall to trigger the appearance of those rats.
BTW, learn any skills you wish before convincing Edmond he is a ghost...
You can get back to the pit level if you are past there. The boney monster can be found east of the kitchen, then south along the corridor, then east past the 2 doors controlled by levers that you have to split your characters up to get past...
After the pit level there are a couple maze levels (the second smaller than the first) and detours, then another small area and finally you get teleported back to the pit level. At that point a previously locked door will be open, and you can take some stairs up to the starting level, and another set up to the citadel.
The quest reward for helping the imp is quite handy, so if you are not there yet, or can backtrack, I would recommend getting it sooner rather than later. It can be left until later, though, up until you move on to the next act.