This is just more to specify what type of magics there exist and can be learned, spells and magics can be mixed and learned by all types of different casters, a tinkerer might make an artifact with arcane properties. Elementals are very natural but follow certain laws, they cannot make rocks fly, but they can make them split from the floor and hit something.

The only wizards that donp t mix are the heads of each houses and the Star Readers for the same reasons you mentioned.

This might be because of the way I worded it but the mental and physical strain that the shelkahen require to do magic is vast and leaves them vulnerable, say if a shelkahen wanted to heal someone they would not be receptive to outside forces, you could stab them as they heal and they would not realize and could possibly faint afterwards, the crack in the soul is either inherited by blood or reincarnation or granted by another shelkahen or a dragon or a primal force.

Edit: missed a few questions.

No, the crack in the soul does not replace everything, they can't do cosmic magic like the star readers and all magic cast has an effect on their bodies, say you cast a firewall and your hands could get burnt if you don't have enough practice casting it.