What are the missable equipment/items in Bravely Default

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.

You get the summoner class in Chapter 2.

It is about two or three boss fights in.

The Summoner class is not all it's cracked up to be unfortunately. You have to collect the summons and at the time you get the class you only have access to two summons.

How to get summons:

You have to talk to the Owl looking guys that you may have noticed. You just have to talk to them and survive one hit from the summon and then you will earn the right to cast it. They aren't particularly strong unfortunately.