Aggro is a problem with conjuration if you're not careful.

That's a very dishonest assessment. While doing normal questing you are not continuously casting spells. Mobs die, then you have to find another...

Destruction is a very slow one to raise. Burning Ralof continously is far from slower than burning random mobs while questing.

its acutally easy to powerlevel destruction. the entchant three is so stupidly op currently that every magic based skill is a complete joke to level.

make yourself an trash armor with 100 entchanting and simply make 4 pieces with two schools you want to train. get into combat without the enemy beeing able to reach you. start endless manoless casting. done.

to level destruction make a set which reduces manacost to zero then go and do the quest for the joining of the companions. when you are outside with vilkas simply cast spells on him instead of using a sword. he wont attack you. his hp will regen ultra fast this way you get destruction to 100 in under 2 hours. try it.

the only real that is bs to level without using glichtes or exploits it really speech. but well only the last 10 points are a problem because theres trainers.

There's a buggy way to slowly, very slowly gain destruction. If you are jumping up and down in a forge so that you can heal the damage to gain restoration, you will also get destruction gains. Although not much. In going from 21 to 51 restoration I went from 62 to 64 destruction. That's a weird bug.

Regarding the oakskin/flesh method for alteration and the extra gains while casting in combat; is it better to let the spell expire on its own before recasting, or recast immediately?

Recast immediately.