One fix would also to:
1/ Handle drops only after Acheron dies. All of them.
2/ Give stones rewards based on the number of bosses you defeated. 2/4 bosses only? 100 Philos and 25 mythos.
3/ Add a "join in progress only" option

This allows people who want to help / don't have 1 hour to make the full dungeon, to still be able to progress / help, while in the mean time removing the purpose of quitting when you got your drop or to only accept runs in progress to make quick stones.