You don't need to know who the attacker is. It doesn't really matter who it is, the coin and value of the coin would be severely jeopardized regardless and so the community I think would easily agree to pass a hardfork deleting the attacker's currency.

Could the government convince everyone to willingly give up control of the currency to them? I kinda doubt it, but that's not a fault of PoS if they could. They could take over the crytpoeconomy regardless of the consensus mechanism in such a case.