I was wondering about what the rhyme or reason is to what abilities you can use as a multi-class character from cross class skills(abilities).

Ok, I made that a bit to complicated... So here's an example:
My character is 5th level mage/6th level warrior. As of now I'm able to use my fire bolt and enhance fire bolt as a warrior, but none of my other mage skills. Conversely, I'm not able to use any warrior abilities as a mage...

So, what's the point in multi-classing if I may ask?

Also, I'm no longer geting training points if I go back and level a class that is lower than my highest... Does that mean I'll only ever get 100 levels of points to spend?

I'm looking at getting to Knight of Creation, but after that I was thinking Druid for the buffs and auras... If I can't use any of them, then what's the point in leveling a second/third or fourth class?

Perhaps I just don't understand the way the game works...
Thanks,
Quiiliitiila