There is a simple way to identify "lost" addresses, you change the address format then give everyone one year to move their bitcoins to a new address. Those still in old addresses after the year go back into the undistributed potential to be mined.

As to truncation that's simple, old non-valid ledger entries (ones that no longer contain bitcoins) get compressed. I would say we're a long way off from any inability to store the blockchain easily on a personal computer, I would argue most computers can hold it in RAM, hell most Android devices less than 2 years old can handle it on a Micro SD.