Okay, I now understand what you were saying. Use Shadowkeeper to add enough XP so she's Mage 10 temporarily in order to enable thieving abilities, and then remove the added XP/level down right before you exit the dungeon. FWIW, last night I decided to see how just how hard it would be to get through Chpt 1 with Imoen "imported" as Thief 9/Mage 4. I got it done, but it wasn't easy despite going straight for the portal key to grab Yoshimo ASAP. You can survive most traps okay and force open locks just fine, but, as I'm sure you know, getting the portal key was very hard. The room traps are bad enough, but that chest trap is deadly - at least on core rules. My PC (8th lvl Berserker w/ 19 Con) barely survived since casting Slow Poison successfully is a coin flip. Maybe it's not as bad on "normal."Still, we were thinking the same thing, leveling Imoen to a 9th level Thief of whatever kit you give her... then dualing her to a mage and continuing the game. But see, the BGT doesn't have a set XP cap like the BG1 TotSC had, unless you use a mod with that feature... so Imoen is likely to reach more than 9 Thief level and 4/5th mage level...
In retrospect, although its a bit of cheat, I probably should have done what you suggested since it wouldn't make any difference in the long run. Chapter 1 is really a prelude to the real game so just get through it, set remove added XP/levls, and play on. Oh well, at least I'm done with it.
What comes to the rest, we have found out that the game has sometimes hard times calculating the XP amounts the characters have, and my suggestion of temporarily setting Imoen XP with a cheat so she gain the Mage level 10 and then using it to reduce the XP total back... just before you exit the first dungeon in BG2, will reduce the chance of you getting a (9)/20th level Imoen as her thief level is already reactivated, so it's calculated as 9/x.
Yeah, exacurated a bit about the level 20, but still...

I have a save right before exiting. Ques: Should I use Shadowkeeper on that save to raise her up to Mage 10, search for some traps or hide in shadows for a few seconds so the game registers her using a thieving skill, save, take out the added XP/level her back down, save again, and finally exit the dungeon and play on? Is that the correct procedure? I'm thinking it's better to address the potential problem now rather than have to level her down upon release and guess her appropriate XP/level.
Thanks for the warning. I might not have thought of that problem though I have no plans to dual class this character. Even if I was going to dual class, I can't imagine waiting until that late in the game - at least playing on core rules or harder. It's essentially what I was trying to avoid in the first place with her. To have her as a 5th or 6th level Mage with no thieving skills available u]and[/u] my character nerfed for quite a while as I level up?! Not looking for that much of a challenge.What comes to the amount of XP that's given to Imoen at the Spellhold, it depends on your characters current level, so if you dual your character just before you release Imoen, you are unlikely to actually give Imoen any XP... so you probably shouldn't.
Again, thanks a lot for the help, and let me know whether I should level her up right before leaving.