dtgreene: Here are the options for replacing the thieving skills:
Open Locks: There's the Knock spell, and there's the option of forcing locks. (Note that Strength + Draw Upon Holy Might (in that order) should (unless the EE changed something here) allow a cleric (even someone weak like BG2 Aerie) to get 19+ Strength).
Even Dorn (who comes with a 19 Strength) can't open all the locks unaided, IIRC. Pretty sure forcing everything requires at least the use of the giant potions regularly (and are there some for which 24 isn't enough? I haven't done exhaustive testing, of course).
dtgreene: Find Traps: There is a spell that will do the trick, and Monks, I believe, can find traps. Unfortunately, neither option helps with disarming the traps. (Can you use a summoned monster to set them off?)
This probably depends on the specific trap. Some traps can be triggered multiple times, and I'm not sure all traps can be triggered by non-PC objects (there are apparently at least a few that definitely can, e.g., the Phase Spiders in one room in Candlekeep catacombs have a Web trap in their room that they spring, i.e., it even says "Trap Sprung").
dtgreene: Pickpocket: No substitute, but Bards (if you can find one) get this ability. (Do you even need this skill anywhere?)
Never absolutely needed, gets you some nice items though, especially if it's an NPC you don't want to kill (or kill right *now*). There are also cases (not sure if the EEs nerfed any of them) where you can get double items from an NPC by pickpocketing an item off them, which is then also given as a quest reward later.
dtgreene: Move silently/Hide: Invisibility spell/potion.
Just about true, and the latter are even more reliable (e.g., just try getting Safana permanently when she is still level 2 without the expedient of two invisibility potions!). On the other hand, potions aren't an infinite supply, and while spell slots effectively are, resting opportunities can be an issue if you need repeated uses.
dtgreene: Detect Illusions: True Sight/Seeing, and I believe there are a couple lower level spells that could work at lower levels. (Note that it is unlikely that you will need this in BG1 as the skill wasn't even in the original game.)
Pretty sure these are all Mage spells that require the Mage to be in sight range of the illusions to be dispelled. As the illusions one worries about at this point are mainly invisible monsters and mages, it's debatable if this is really a substitute (the Mage is much more vulnerable, and also has to be visible to these enemies, as spellcasting will dispel their own invisibility if they have it; the Thief can be stealthed while exposing illusions).
On the other hand, there are not nearly enough Thief points, especially at low level, to be allocating to something as situational as this ... (Mages, human or Ogre, and Sirines are about the main enemies that use invisibility)