Actually ..
I dualed at level 16 too
Remember though, that this means, your mage wont reach a very high level
(XP-cap), because the XP is summed up. I cant remember anymore, but you might have to remove the XP-cap (not so good choice) or use shadowkeeper to get to atleast lvl 17 (so thief skills kick in again). The reason you shouldnt remove the XP-cap is because you'll easily be a lvl 23 mage that way, while the ability-bonusses only stretch to lvl 17, it gives you 6 non-counting levels and that's NOT good if you want to play ToB.
Allocation:
Pick pocket is a definite MUST here indeed
I suggest you get your Hide in Shadow to around 100 too though, just make sure, that -once buffed up- its around 150.
Find traps you do NOT need, not for the XP, you'll get plenty as a solo, and NOT for the HP-loss since there will be none, if you have
stoneskin + mirror_image
Lock picking is optional, but there's allways the "knock" spell (no XP though)
Forget Detect illusion too, it never worked when i tried to use it, and i spend
!!200!! on it.
Basically make sure that Hide in shadows and PP are your main concern and you'll be fine
Improved invisibility will NOT insure unlimited backstabs, the enemy will see you once you attack. Potions of invisibility, invisibilty spell and the staff indeed work great in a battle >
Do make sure you have the cloak of NON-detection on you later in the game,
because some enemy's have a natural ability to see invisble creatures.
Just have it in your inventory, and wear a more usefull cloack most of the time
Hope this covers it all