As far as I understood bitcoin, there is no authority. Nobody can do anything about a user having 500k of his coins hacked, and nobody can do anything about the market crash either.
As long as the money hasn't been withdrawn from the market, the market should be able to reverse all the transactions. The problem would be if the BTC or USD were withdrawn from the market before they shut down trading.
MtGox != Bitcoin. Transactions on bitcoin network cannot be undone, that is true; however, MtGox will undo the transactions inside their system (buys and sells within MtGox). The only thing that cannot be undone is the withdrawal of 80 stolen BTC from a MtGox account into Bitcoin network.
As far as I understood bitcoin, there is no authority. Nobody can do anything about a user having 500k of his coins hacked, and nobody can do anything about the market crash either.