Not sure about the fondant qu- do you mean the rolled icing?
Anyway, I would support it JIC!! You can buy thick wooden skewers from cake shops which you insert into the bottom layer of the cake and then cut them off at cake level before placing the next cake on. Often the next cake is placed on a board that is smaller than it (which u can't see) and which sit on the levelled skewers. The skewers are about .5cm thick which I would recommend as mud cake is quite heavy.
Would you try a chocolate ganache as an icing?