Jumping in with an idea―maybe when claiming missions people can add in their typical tagging rate/speed like they do on CR memes? So people who want to tag fast and might lose interest in the thread if it takes too long can be matched with other people who feel the same way, and people who want to take things slower and backdate can do the same. And obviously if extenuating circumstances come up, they can inform the other group members to work things out.

Since generally multiple parties can take on one mission, I don't think this would exclude too many players who want to get in on a specific one and it would give people who need to be slower but will backtag the ability to participate in missions.