If you do not bring a teleporter stone into the castle with you, the imp historian just shows up the next time you try to use one (or rarely they simply stop working until the point you would have gotten them back).
The exit from Iona's dungeon is on the far west side of the first level. The lower levels (opened by reading a scroll, IIRC) are optional, and you can return for them later if you wish.
Iona's dungeon maps:
level 1,
starting cell level and levels 2-4Welcome to the forum.
