So about Commemorative Coin.. Official answer please :) ?

[REDace0]

I speak from experience on this, if you don't have the space for the seals, the game won't let you make the exchange. There's no way to lose an item. Otherwise, I've have lost a cluster.

I'm glad that's the case, but the event turn in isn't exactly the same as turning in a quest, so the same check may not apply. It depends on where in the code the validation logic is located.