Maybe a solution to this could be that the event doesn't end after the amount is received. Make the amount to be collected more, so it could be harder but also let people participate even tho the required amount is fulfilled.
For example, the last world quest started at 00:30 on my timezone, I collected almost everything needed but still was missing a pokemon or two and because my eyes were closing I thought I"ll catch the final after I wake up and then submit them. Then I woke up and joined the game 15 minutes after the event was over.
Another solution could be with the 2 world quests every month, and made them at different timezone trying to find balance. And people who successfully participated on the one, to not be able on the second, with a message like "You already submitted for the World Quest this month." .