I kind of feel like the game screwed me by taking away Yoshimo. I no longer have a good theif/scout, because neither Nalia or Imoen are good at it or even able to gain more thief levels as far as I can tell. I understand Imoen is central to the plot, but right now she's just a worse version of Nalia. The rest of my party is Minsc, Jaheria, and Aerie.

Without spoiling anything, how should I proceed (I'm currently in the first area of the Underdark)? I've been thinking about using a save editor to turn Imoen into a pure class theif and buff her stats to the levels Yoshimo had before he was taken. Is there any risk of screwing somtething up by doing so?

Imoen is perfectly capable of handling *any* thief issues you have (except back stabbing), and she's infinitely better at it (level 7 vs level 4) than Nalia. She is also equally adept as a mage (both have 17 INT). Imoen's only downfall -- she has the best stats of any NPC, bar none -- is that she's forced out of the party for too long, meaning you have likely given all of the best scrolls to Nalia. With careful planning, Imoen is a far superior Mage/Thief than Nalia, but you need to get to her quickly, foregoing side quests until she's back in the party.

Your question was about thieving skills, and Imoen is the best there is, NPC wise. Go with her; get her some decent spell scrolls, and you won't be sorry. There is absolutely no need to use an editor. She's as good as it gets.

Fair enough, but she still seems terrible stealth wise. Takes forever for her hide in the shadows to suceed.

Yes, there had to be a compromise with Imoen being dual classed. She was given a lockpick and find traps role, to get you through Chateaux Irenicus, with a token 10 points in Hide in Shadows, and Move Silently. You could always reassign her thief points, especially if you intend to keep both her and Nalia. Nalia could be your lock picker (she's not much use for anything else, thief-wise), and you could put Imoen's skills into traps and stealth. This is easily done, if that is what you want. You will need Shadowkeeper. If you're not familiar with it, be careful!

Thanks for the help!