Nalia will make it, but you should plan on having a few potions of Thievery and a few knock spells memorized. You might also want to make sure the Aerie has a Find Traps spell memorized.
If you pick up Jan at this stage in your game, his level will adjust somewhat to the level of your party. He probably won't be as good of a Mage as Nalia, but his thieving skills should be better. An advantage of Jan is that as he levels, you will be able to adjust his thieving skills, where you are stuck with Nalia where she is at.
You have pretty strong magic support, so losing Nalia as a spellcaster isn't going to hurt your party much.
Also, keep in mind that characters get experience for memorizing spells and disarming traps. You can always release some of your party members (let them hang out in the Copper Coronet) -- save first in case if something goes wrong. When your party consists of yourself and Jan, have him start memorizing spells ((have him drink a potion of genius if you don't want to miss any of the memorizations). Rather than divide the experience 5 or 6 ways, you each get half. He should level up pretty quickly if you have the right spells.
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Most Subtle Official Straightman of the Laughing Hyenas -- Grammarian Brigade.