Well, there may or not (?) be some problems with some aspects of this event, but, enough babbling, how would you "fix" the event?
Here is one quick suggestion:
There are five chests in Bonus Encounter of every difficulty, a random one will drops a certain amount of Unrefined Gems + event itens, 17~20 Unrefined Gems on Legendary in this case.
Fix:
Make all five chests drop Unrefined Gems, 4 drops on a low amount (3~5, don't drop event itens) and the random "chosen one" keeps dropping the same what is already dropping (17~20 Unrefined Gems + Event Itens) .
This way, people that can only kill 2 chests can gamble a little and don't feel like wasted their time by not getting any Unrefined Gems for not killing the "chosen one"
With this change people that can't kill all chests will at least be able to purchase more in the bazaar, the ones that already can kill all chests, well those deserve better trade ratio in the bazaar as its already being discussed here:
viewtopic.php?f=6&t=1200
Edit: Bad Typing and some value change