Im posting my progress, as this is my first time doing this event, and Im curious if my experience is typical. Counting the surprise box and the last upgrade from the daily quests, with 7 days to go, I am 1 upgrade away from getting the full upgrades to the Ferris wheel. To get there, I will need either 37 more idols, or complete the remaining days of the calendar. I have a gem uncovered for tomorrow, and 6 gem shards. However, I only have a few tools left, and only 35 scrolls, so it will be 2-3 days before I can get more tools, so filling up the calendar look unlikely, but getting the remaining idols looks barely possible. I have managed to do all of the tablet quests except for the gold one (have done some of the common ones 3 or 4 times at this point, for the extra scrolls).
So it appears to me, like the spring event, it is possible to just barely manage to get the full upgrade of the event building without diamonds, but only if you play the minigame very efficiently and dont make any serious mistakes.