How is Proof of Stake useful in an interbank network though? My understanding of PoS is that everyone wants to follow the same chain because the interest payments on your stake make it beneficial to you to accept a new block. Presumably there wouldn't be anything to stake in a private blockchain.

Proof-of-stake is not a consensus algorithm on the same level as proof-of-work.

When faced with two blockchains, one in which I control 100% of coins -- and thus 100% of the stake -- and some other chain, which chain do you choose, and why?

You can't use stake to make the choice, because that's a property of a chain, so you need to decide which chain to use before stake has meaning.

Also, what happens if an attacker exploits a bug in the Ethereum implementation, and uses it to replace all nodes' chains with his own copy? How can we ever return to the correct chain unless we just use trust/majority vote?

Actually, if you read one of their blog posts from the last few weeks, they've pretty much abandoned it because it's too hard a problem to solve. Sorry I can't cite, on a mobile and in a rush.