Thanks for getting the discussion going!
It's sure that nature has elements as well, and globally Nature Magic will get the classic Air and Earth spells in other game systems. The division between elemental and nature is done because there were too many spells relying on elements to fit them in one school, and also to better characterize the Mage class vs the Druid class (I'll talk more about how magic schools and magic user classes are related in part 2 of the post).
Light magic is also the color of enchantment, so it gets weapons/armor buffs. Open locks? That's a good question. Mind maybe, as it's a question of mind controlling/altering matter.
3 quick slots because it uses the same user interface as the weapon attacks (which had 3). It seemed like a good amount to force you to make choices about what you want to be readily able to cast.
Reagents for normal spells, I wouldn't say so. But maybe some other special kind of magic could require them...
