The second is how do you ensure the summary is accurate? To verify the summary requires having all prior blocks anyways.
Good point. If summary disagrees with full history, which one is authoritative?
In current system, transactions only get into block chain once and keep ageing. Old transactions thousands of blocks ago don't need to worry much about current block chain attacks. With summary ledger, they can be messed with at any time.
Current system, only a previous owner of the coin can mess with it. Even under 51% attack, only previous owners of coin can try to double spend... With summary ledger, we take some miner's word for it.