First pass without trace is useless in this situation for a number of reasons. The first being the Imp is for all intents gone. He could have left, gone invisible, is hiding in the room, or one of a dozen different things, so sneaking up on him is at the moment impossible as you don't know where he is at. Also pass without trace lets you move over ground and leave no tracks and such or any sign you've moved through the area, it does not grant you invisibilty. However the area is full of rats and they will see you and feel you as you try to move through them.
As for spells you have to sleep/rest/pray a max of 8 hours to regain your spells, 4 hours if you use the spell of Nap.