Yes, you have all 8 keys. And yes, you can’t access floors that haven’t been built yet, even if you have the keys to not-yet-existing floors.
As players of your region, including you, have donated enough sandstone for the first five floors, you can access them with your keys.
Now you have to wait and help with donating sandstone until the next floor is built.
Hence the name: Builder event.
100% is eight floors, so each 12,5% opens one floor. Your next floor opens when your region has reached 75%. Next 87,5%, then 100%.