Ever since we were told the last boss is Garrosh, it became obvious that we are not going to fight a normal Garrosh. It is the last boss fight of the expansion and it needs to be a little more epic than fighting an orc using warrior abilities. He is going to be empowered somehow, probably from Sha or old god entity. It is indeed possible that Y'Shaarj might attempt to use Garrosh as a new body, and the two will struggle for control. Inhabiting such massive amounts of all kinds of negative emotions might lead to Y'Shaarj's rebirth. I expect the boss fight will have multiple stages where we will fight Garrosh's negative emotions: Anger, violence, despair, fear, doubt, hatred and finally pride. Pride will probably be a heroic only stage, or the fight shifts from fighting Garrosh to Y'Shaarj.
Definitely parallels to the prison in Ulduar. The Titans have a way of doing things when it comes to sealing away the Old Gods.
What you mean chains that shouldn't be broken? When the box obviously contained the heart of the old god thing since It's under Garrosh's possession now obviously It's going to have something to do with Garrosh in terms of him being more coo-coo I mean really It's quite obvious but SoO isn't out yet so I can't really say.