"The Last Quest Requires receipt of a Mystical Treasure Chest in Tower, but Tower never provides them. So how are Players expected to Complete Final Mission?"
I eventually found one, after lots of grinding tower fights. Unfortunately it has a 36 hour countdown before you can open it and I got mine at 6 hours until the end of this current event. It looks like the only way is to keep fighting and fighting until you find one, then keep it for the next event, if you can't use it in the current event.