I don't think your first post showed up. Did you maybe edit it, and hit the 'delete this post' button by mistake rather than the 'change this post' button?
Anyway, no you do not have to take the stairs down that were opened by the scroll. The exit is to the far west on the current level. The lower levels are optional, and you can wait until you are stronger, or at least have your teleporter stones back, before clearing them out.
If you have any shadow potions (the cook in the castle gave you one if you washed some dishes) or a frog statuette you can run past a lot of the monsters.
Help! Can't find Gyrion... Iona's dungeon maps
If you have a save before entering the castle, you could also reload and wait until you have cleared most of Rivertown and Verdistis before entering. That is what I did, and I didn't have any problems with Iona's dungeon. You can just keep going as well, though you might have to level up a bit before you can handle the area where you get your teleporter stones back (scorpion traps or a weapon with Frost on it would help).
Trying for the lowest possible ending level, Ariana went through Iona's dungeon around level 5, hit the gong at level 12, to end up in the wastelands at level 17, then quit.
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