There is no such thing as "spent" and "unspent" transactions in the mini-blockchain scheme, and you're right, it's not the same as a "rolling root" design. The transactions simply perform operations on the account tree (the balance sheet). So you see, the inputs and outputs in our transactions do not point to other transactions, they simply point to addresses in the account tree, so the transactions aren't linked together the same way they are in bitcoin, and we can discard all transactions after a safe amount of time has elapsed (enough to make the Secret Chain Attack infeasible). In our implementation nodes will be able to delete all transactions older than a week, but they can choose to store as much history as they want. I'm sure many people will even choose to store the full blockchain for historic reasons. But for new nodes who don't have any of the blockchain, they wont need to download the full blockchain to synchronize with the network and become a full node, they only need the last weeks worth of transactions and the account tree. Of course the account tree will grow over time as new non-empty addresses come into existence but the level of compression and scalability this system offers is virtually unbeatable.