"Can the drive be accessed over the internet, e.g. using a MacBook away from home to browse a documents folder on the Time Capsule drive?"
Yes, it supports Wide Area Bonjour, the basis of Back to My Mac. However, Apple hasn't made it obvious how to connect the device to its .Mac servers in order to do this. Savvy users with access to their own DNS-SD server will be able to set this up themselves, but the rest of us will need to wait for Apple to make things more obvious.