Maybe it's the female human mage animation or paper doll that it's trying to switch to that's corrupted.
Unlikely, but remotely possible. An easy test would be just to create a human female mage from scratch (don't bother trying to dual class).
Now if a thief was dual-wielding at the time and the mage animation or paperdoll *can't* dual wield, I suppose that could in theory cause a crash. But I doubt that's the case since all sorts of folks would be reporting it even in the unmodded game.
Edit: here's another test. If you can create a human mage as above, then try leveling her up 1 level at a time with CLUAConsole:SetCurrentXp. Any time you get a spellbook selection and it crashes then, you can narrow it down to a particular spell level and hopefully a particular spell graphic. Try it with a sorceress instead of a mage (same animation I think, but gives you more spell access).
The thing is, I was able to dual class a mage successfully, if I started as a fighter instead of a Thief with same initial spells and everything that was crashing for the thief, but not the fighter. Plus, in my tests for the thief dual class, I completely stripped the character, so he/she was wearing no items.
Honestly, I think it seems to be specific to the Thief dual-class option, because it crashes on either Male or Female, the more and more I think about it, the more I see a paper-doll transition issue between a Thief to a Mage when it's dual classed. (I'm not even sure if that's possible or makes any sense though) So, is there any console cheat I could use that can dual class the character without going through that dual class screen itself?
In the end I think this must be some fluke bug, I was very careful in picking what to install, I did not want to do the minimal installation, but at the same time I wanted some of the nicer expansion mods too, and ironically, it has been my best playthrough so far in terms of that this was the only crash I've experienced. As for example, I had that huge game killer rest bug (where anytime I would rest, it would crash the game, another rare bug that no one seems to get, lol), and it seems to gone. (Although I am very careful in creating multiple saves, so perhaps that's helping too, but who knows..)
Also, as for the Debug question in the BWS, I'm sorry, going through all those files is really hard for me to understand, therefore, I attached it (compressed format) to see if anyone would like to take a look at my BWS Debug.txt for those more seasoned towards those kind of files and pinpointing issues.
Naturally, this is not game-breaker, but I do appreciate all the help with this, so thank you