If you’re just handed a USB drive you don’t know if the other party also has the private key.

Right I don't know if there's a way around that. I assume you have to have the mint communicate with a device in such a way that the device can only be "unlocked" once? At first glance it does seem "possible". But if you're handing someone some ecash, they can just redeem it immediately, for Bitcoin or for new ecash.