Really, nobody giving comments on this? From my view, the stats do seem to be fixed into 3.5 in a reasonable manner, and I agree with your reasoning concerning the value of HD and class levels. I am not sure how the default rules handle all this, but I can safely say that a HD that gives nothing is not as good to a player as an actual Class Level, like you said.
For the succubus, I am wondering if it be too strong to just have it advanced fully to 12HD, and remove all the LA? I have always felt that ECL12 was too steep for what is essentially a 1-trick pony (party face+charm monster spam).
Though charm monster at-will can be abused to some extent, since you could theoretically charm an entire army to be at your beck and call. How would you balance this in-game?
Sorry for beating the dead horse back to life, but I couldn't help but notice that nobody bothered to reply here. And what do I have to say? I can see that clearly you have put an effort into making these monster classes fun to play for Players, and balanced them as such.
I don't really have anything to say other than that, though.