Sub-Quests reset in chapter 5

Having tried to build a tank in that game, I can tell you it doesn't work. At least, not the way it would in a MMO "holy trinity" type of game that most people think of when creating party structure nowadays. The items that make the equipper more likely to be targeted only do so by a small amount, certainly not enough to be noticable in my experience.

To answer your question, the enemies will decide who to attack on their turn. That is: when they're about to go, right before each hit, assuming they don't brave. They are also reactive to what your characters have done, not to what they are about to do. If you queue up a 4-brave healing spree, the aggro is generated when those heals are cast, not before.

Granted, even with the aggro, there seems to be a random factor involved to prevent "perfect" tanking. Because of this, the game seems to encourage powerful healers and group durability more than glass cannons and solid tanks.

The earliest class able to attack multiple enemies without using MP is the Valkyrie, obtained in Chapter 2. Their initial skill is Crescent Moon, which deals conventional damage to all enemies and costs 1 BP.