Can you put flowers and things made with royal icing in the freezer? And if so, how long?

They will keep in the freezer, but the risk comes from the condensation that is likely when you thaw the items. If they are in a sealed container, and you thaw them IN the container, the condensation forms on the container. I would test to be certain.

Of course, Just flowers (not on a cake) and decorations do not require freezing. Once dried, they will last 'forever'.