Obtaining the Display Case and Sugar Skulls without cheats

I would not say that the items are permanently missable, because that requires a content state of not being able to get it at any cost past the time line. The challenges are seasonal and it is more often than not more likely to find the same challenges active around the holidays. In all do respect that is better than the permanently missable items you get from limited time features MMORG missions. I believe the surgar skull and day of the dead challenge was always meant for fall, and the #Trick or #Treat was a test for the internal system we got to participate in. The Egg hunt will return around Easter and the surgar skull will return around fall. So it is highly unlikely any permanently missable unlockable items that I know of.