I think you're going to be able to continue questing after the seals are closed to make the rest of the tokens you need and when an item is made the seal counter (if it goes above 100) is reduced by a certain number (maybe per item made?).
I'm going to wait for two things before I weigh in on this. We need a proper write up of how the quest works so that things like the seals are clear for everyone. Also need someone to actually attempt to make an item, it's one thing to go down during test week and try to make an item, but it's another when you go to make the first and every person in a guild with a fort (and some that dont) are going to be down there trying to stop them.