3b) Going in as a healer is pretty simple, but you will find yourself switching between healing, dps, and target management. For example here the Tonberry King is a difficult fight, because the tank likes to chase the little tons and loses aggro on the king. I got around this by taking the little buggers out myself before the tank got distracted, and releasing a well timed party limit break.
3c) As a dps you are primarily keeping the squad focused on what groups to attack, and paying attention to which mobs they are focused on to make sure you don't steal aggro. While this may seem a safe way to approach this, there are plenty of ways this can go wrong as mentioned above.