1. Destruction / Enchantment, grind enchantment up as quickly as possible, enchant your gear to remove the spell cost from destruction. (Breaking the game this way is the only way for Destruction to stay viable)
You don't need to do this, it's so boring having 0% magicka consumption, makes the game to easy. I use ~90% magicka consumption, so I can cast a good few spells but not endlessly stagger the enemy. I play on master, using destruction and I don't feel the need to have free spells, I like having to plan using spells so I don't run out of magicka. Or maybe kiting around the enemy to restore some magicka.
3. Illusion / Something Else, Illusion is very powerful because you can make enemies kill each other, however, until you hit 100 Illusion your spells won't affect many creatures, so you need something else.
Wrong again, I'm level 59 and I dual cast the basic fear/calm/fear spells on everything that isn't immune (dragons etc). You just need the right perks (on the right side of the illusion tree).
2. Conjuration / Archery, it still takes freaking forever to kill anything with summons alone, but bound bow gives you a decent attack, and this build doesn't run out of magicka without enchanted gear.
Wrong. Conjures get the same bonus to health/damage that enemies do when you change the difficulty. On master they are very powerful, when you get the expert spells and the master spells, you can sit back and watch your conjures do the work for you. To the extent of conjuring 2 dremora lords, and they clear a room of draugr deathlords on master.
So yeah, your wrong.