Ok not all. But the master spells with area effect are actually dissapointing. Trying to cast them during battle is difficult and they target your companion too.Overall i never use them

Likewise. I depend on high conjuration for a pair of demora lords for most of my damage, with my own girl swinging a 2 hander, Illia for fire support and Barbas for tarpiting. She is only lvl 30 and has only just reached 50 with destruction, but I have -37% to casting for destruction so she throws a lot of fire balls or lightning bolts.

I'm not sure about a pure destruction mage, but with conjuration, restoration and alteration, a practical pure mage is quite possible.