Does this mean that the only way to fully drain 100% of your mining earnings from the pool is if the pool encounters a string of long blocks, forcing the pool to pay out your 'banked' earnings?
No, the only way to get 100% of your earnings is to mine for infinity. The only way for this system to be fair is to decay old shares
in a single round like Slush's pool attempts to do, otherwise you will
never get your fair payout as there will always be something held over.