1: If you have max range passives, and STAY at max range, his stun cannot reach you, that is the point of the Range Passives.
2: Having toyed with the aggro system in ROSE I can safely say, no. Mobs in DG don't just go for Mage first. They go for whatever hits them first.
Here's how the AI works for CoU Spiders for example. And you can test this yourself.
1: Spiders are hit or player enters their aggro radius.
2: spiders target and attack player.
3: AI then clears aggro, and begins sequence again. If in this clear phase you hit them first, they will target you. If your team members hit them first, they will target them.
4: Lastly, 'taunt' mechanics will only work for 1 hit, then the AI clears the taunt, and re-selects.
There are obviously other variables, such as apply status downs, range to the target. Player level to monster level, etc. However, the aggro system is pretty easy ti manipulate.
Having more practice doing so will help you survive longer.