Want to decorate a buttercream cake with fondant Christmas decorations and my plan is to place it  in a cake box , wrap with baking paper (like you would be a present) then wrap in plastic wrap and because I am a freak put that in a plastic bag haha. I believe it will be fine but tend to doubt myself. Has anyone had success or done anything similar. 

And was thinking could I not just put in an air conditioned room to defrost instead of the fridge?

I think your plan for prepping the cake for freezing will work fine.  The only thing different is that you are wrapping it in baking paper before the plastic wrap.  It will only be frozen for a short time anyway, so especially with the plastic bag, it should be fine.  It should also be fine thawing in a cool room.  Just leave it completely sealed until the whole thing comes to room temperature and any condensation has had time to dry.

I would suggest testing it first -- it probably will be fine -- 

but how long will it be in there? is it a frost free freezer, is it opened regularly by family to get ice and  stuff? what kind of fondant decor, some kinds of fondant decor would wilt -- under certain circumstances red food color can bleed -- and I would only defrost in the fridge, myself -- and I always delivered cold cake

Sandra’s advice is spot on :)   Sorry I  just now seeing this post.   By now you probably have it nestled in the fzr so just know yes an air conditioned room is fine.  Leave it in the closed box to defrost.  It will be fine to open it when the bottom of the box is cool — nt cold:(