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Can we get a definitive answer as to whether these can be completed later within the 14 day period?
Yes, they can be completed later while 14 Days Of Fortnite is active. Each day during this period a new challenge unlocks, along with its specific reward. Once all 14 days worth of challenges have been unlocked, there will still be a few days to complete them before they go away (IIRC they expire on either the 3rd or 4th of January).