My daughter wants an ice-cream iceberg cake with arctic creatures on it, so I covered a board with blue fondant for the sea, am making fondant penguins, seals and polar bears, and am going to make the iceberg out of icecream and put it on the fondant base with the animals on top. Can I do it in advance and freeze the whole thing, or should I do everything and put together with the icecream at the last minute?

You can freeze MMF, it gets rock hard when you freeze it.  I don't work with any other kind of fondant so I can't answer for those.  So bottom line is as long as people know they might be rock hard to use caution if they eat them, you should be ok.

AI made marshmallow for my base when I did a penguin cake. I made rice crispie treats hand molded while still warm as the mounds of snow, I covered the entire board with the home made marshmallow which is easy to colour. You can freeze marshmallow. On the day finish by putting the animals in place.