"Exclusive" does not mean "omnipotent". We negotiate "exclusive" agreements all the time. The fact that someone else agreed to give you that exclusivity doesn't make the "exclusive" part go away.
If not "Unaccountable", accountable to whom? Sure, there may be other individuals in the hegemony who can raise a flag, but its not you or I. The NSA can lie to the senate and it takes a brave senator to break ranks but even then all they can say is "I think the NSA lied on one of these points" but can't even say which one. There is real coercion going on here.
It's not exclusive. Exclusivity dictates that the power Roberts has is available only to him. But it's not; it's available to Congress as well. Not only that, but Congress' power over the FISC dwarfs that of Roberts. Congress could pass a law requiring FISC judges to decide while swinging upside down from a rope made of cheese curds. All Roberts can do is appoint FISC judges. Congress can un-appoint them.
It's not unaccountable. Unaccountable means that Roberts can exercise his power unchecked by any authority. Roberts power is checked the same way as the Secretary of Labor's power is: Congress can require him to testify before a committee and, if it wishes, strip him of his FISC power.
- Congress may revoke the power over the FISC, so it isn't irrevocable.
But the FISC Chief is a lifetime appointment, by definition, since the Chief Justice is a lifetime appointment. It is exclusive, because he has executive authority, he does not have to take advice from anyone before making a decision. Whether it is unaccountable is a matter for debate since the operation of the court, and its activities are carried out in secret. It is certainly unaccountable in the normal sense of the word.