You gained 10 reputation when you entered the castle, so there is a net -5 reputation.
If you have a save just before entering the castle you could re-load and enter at a higher level, if you want. The exit in Iona's dungeon is on the first level (where you get your stuff back) to the west. You can use the health and mana shrines which re-fill after a short time, or run past everything to the exit, and come back when you are stronger.
To get your teleporter stones back, you need to leave Iona's dungeon, rescue Zandalor (castle treasure room, from the Rivertown sewer entrance), let him teleport you to the council hall and clear it of monsters.
I did most of the quests in Rivertown and Verdistis, and cleared the Rivertown sewers before entering the castle. It is do-able at low levels though (Ariana entered the Castle around level 3, cleared the council hall around 7, and rang the gong at 12).