1. I restarted the game with a new game and using only the Start pack of stamps did not reproduce the glitch, so I'm not sure what happened then. Given that it only happened once in a long game, perhaps it would take a large number of stamps to reproduce the glitch.
2. because of the sequencing issue in (3), I tend to open Mystery only if I must, so I would leave the unopened Mystery pack on the desktop, but this pack would only last for two at a time. Purchasing the third pack still results in only two packs, but I have paid for that.