2.- The relatively high confidence that the private key can only be used by the owner of the coin.
Is there any indication the German authorities want to see block chain records? I'd expect they want to see bank transfer receipts etc.
In any case, any cold storage wallet will satisfy 1, and offer better confidence only the owner can use the funds. Don't get me wrong, Casascius seems like one of the most trustworthy people here, but I still think it's bad protocol to trust anyone with your private keys if you don't want them to have the ability to spend your coins.