Now in Act 4 ... from the Judgement room, SE there is a key. I had dk go into the other room and open the door s of the se room ... the key doesn't work there - but then, none of the pile of keys I seem to have accumulated work anywhere. Is there anything important in that cabinet or can I ignore it? There are an awful lot of locked cabinets and doors, none of which respond to any of my key pile.
SE and SW room each have a lever - split your chars - these levers open each south door. The SE lever opens SW, SW lever opens SE - there you will meet an imp and get the key for the SE cabinet you need for the mirror (correct diamnod ring is in there). The key on the table is for the SE exit room. the one you took and eventually reached the imp hole room.
So, no bug.