I liked number 15. I like number 16 and 17 too. Finishing this contest in time is possible without diamonds, but it is a challenge and it requires a lot, but the reward is good.
About number 16
You don't need 4 buildings. You don't have to upgrade 4 buildings to level 5, that isn't what the quest says or needs.
The quest asks you to do 4 building upgrades to level 5 and higher. It doesn't say 4 upgrades with different buildings.
I just tested it, upgrading one building again after reaching level 5 and that counted.
So one building is enough if you want. You can prepare this one, build one residence or armory or another building of your choice to level 4 before you get this quest. You can already start the upgrading to level 5 when you are doing quest number 15, but make sure you will finish quest number 15 before your upgrade to level 5 is ready.
When you get this quest number 16, level it up to 5. That is one building upgrade to 5 or higher. Then level it up to 6, that is building upgrade number two to level 5 or higher. Then level it up to level 7, that is building upgrade number 3 and level it up to level 8, that is building upgrade 4 and it will complete the quest.
Of course you can choose to level up 4 buildings from 4 to 5, or 2 buildings from 4 to 5 to 6, any combination is possible, upgrading something to level 5, another to level 15 and such will work too.
I do have a question about quest number 12. That quest asks you to make 3 spells OR scout 3 provences. Of course I wanted to make spells, because scouting 3 provences would take 10 days. I made a spell BEFORE my scout would be ready, but what would happen if my scout would be ready before finishing the spell production? Would it not be possible anymore to create the spell 3 times, because the system would see the scout as my choice because that one finished first? I already know a combination doesn't work. But is it possible to scout 1 time and then produce the spell 3 times? Or won't the spells count then?