What if, as you went in to do gatekeeper and kill Atheon, you had a warlock stay outside the door. This would need to be done without wiping, but what if whenever people are about to get teleported the warlock kills himself the. Self reses so he's still outside the door and when Atheon is killed he goes and looks around for things. Just a thought since you mentioned Atheon could have to be killed to find the chest.