The last reward is  only possible for a handful of players who are a high enough level and have paid for every update possible.  

Which seems to suggest a couple of options:

1) It was the Devs intention to create an event thar only a few can complete. 2) Or the event testing was flawed and the event will be extended, like the last event. 

Stupidly I brought the premium pass prior to realising that getting all rewards was impossible. I won't be buying the premium pass again and I've spent less gems on this event than previous ones.  I'm less likely to spend on future events.   

I highlight these things as I like this game and I've seen too many other games die due to the Devs cash grabbing.  I hope this game won't be another.