When you exit a room trough the door you came in from you will return to the first room where you can exit. But if you keep going trough doors you didn't come in from you will progress trough different room layouts with even more then 13 rocks a chamber.
IIRC Memento doesn't work on Dark types, so wouldn't it be safer to get a Dark type with a status move (Thunder Wave/Hypnosis etc.)? That would eliminate the 1 turn needed to risk switching to the Imprison mon.
After doing a bit of testing, I've found that HA Dusknoir is perfect for this specifically as it has Frisk, Imprison, Memento(Egg Move) and Curse. So if you have the capital to waste you can pretty much guarantee Spiritomb can't kill itself, unless struggle of course.
