The gradual yellowing does sound like light-piping. If it's bad storage, it must have been really bad and totally careless.
(FWIW, I recently finished off the film in a Kodak disposable camera which had been forgotten in the back of a car for at least a couple of years. Expiry date was 2005, shots taken way back were poor, but the current shots were fine. So it seemed latent images suffered but the unexposed film was not noticeably affected).