Perhaps I'm missing something (I'm sure Soggy can help sort me out on this), but it looks like they decreased the number of sky essence that you can obtain naturally (without diamonds) for this event.
Whereas the Eldrasil's event and the winter event both were relatively easy to get the third grand prize by utilizing the "double days," this event seems to only grant you 1160 feathers, quite a bit short of the 1450 goal for the third grand prize.
2788 essence from sequential quests
875 essence from daily quests
250 low estimate of 10 essence from eggs/day for 25 days
3913 sky essence ... round that up to 4000
4000 essence will let you open 28.5 of the third egg ... round that up to 29.
Even if you open ONLY on "double days" ... that's 1,160 feathers, 290 short of getting the third grand prize. That means that to have a chance of getting he third grand prize, you would need 14 of those eggs to contain bonus sky essence. Statistically, you have no right to expect to get more than 7-8.
Sooooo ... this event seems to bring us back to the days of "you're pretty lucky if you hope to get the third grand prize without diamonds." It's not nearly as impossible as it used to be, but they seem to be offering less essence overall than in the past (maybe because there are less sequential quests?), which means getting a full set isn't going to be as easy as one might hope.
I'd LOVE for someone to show me where I made a mistake, because I was really happy when that third GP became more accessible overall.