I dont know the answer to your first question but yes, the prizes for buying decorations are in the event, not the main board....

Decoration events have always had points turn into coins before.

So, I don't really care about the event decorations, I just played to play. If I don't use my event coins they roll over to regular coins after the event, right? I have about 7k in event coins stashed up as I've finished all the big tasks 

And the prizes you get for buying decorations- are those in-event gift boxes or main board gift boxes?

What happens if I don't buy any event decorations?