You keep returning to that NPC thing, which I honestly think is a super bad idea. It's an OCT. It's about fighting OCTs. If I were to fight any NPCs, I wouldn't spend more than a single panel on it, which would be my OC, leaving a dozen bodies behind him. NPCs are good for setting the stage though. There could be plenty of devils, angels and dead people roaming around, but not as opponents.
Otherwise, I think your idea is an improvement upon Cog's (and coincidentally, closer to one I suggested
). I think a good OCT needs a large, free roaming environment open to interpretation, so the idea of people being in different parts of purgatory sounds appealing. I'd suggest that purgatory is a personal space, created by the sins of each OC, and therefore a part of the characters story. Fights could take place in the bridges between these pocket dimensions or on the border between related purgatories. A part of the Character Reference could be a list of purgatories a character has to go through and why.