[As directed by GM Aesthphron]
I have 3 suggestions to improve this scenario of Griefers (or just people with plain bad memory on what turn they were on) locking 7 other people out of Coil turns.
1. Prevent people from joining Party Finder explicitly marked for an earlier turn.
E.g. People on turn 2 can't join Turn 1 parties.
2. If unable to enter because someone is on a different turn, let the party know who that is and where the progress is saved. If a turn has parts like Turn 1, this will matter.
3. If someone on a higher turn enters, give a confirmation dialog to the other players to confirm that they do intend to skip.
Please consider these for other similar lockouts in future as well.