There are a few things that I still can't do well and spheres is one of them. I try and try, but they come out "wonky" in shape and the fondant coverings are terrible. What I have learned is that making the bottom half of the rounds out of pre-made half spheres is the way to go. Cake does not like being upside down so gravity is not your friend. Dallas Foam has all kinds of sizes and I always keep a few on hand for sphere "emergencies".
I've really noticed this year that black fondant can fade really fast. In a day it can go from dark black to dark gray. My first advice is to work extra black color into it before you get going. Someone mentioned adding baking soda or baking powder and that would help, but I can't find the actual instructions, so please let me know if you have a clue what that tip is all about! The weights are all covered with modeling chocolate. I did not have any problem with it fading. The medicine ball is fondant and I had to paint it with petal dust just to get it back to black.
I used the jump rope as a "border" to hide the yucky bottoms of the top tiers. Also on the three weights, I wanted a "lip" but when stacking them, I had to cut away the areas that would have been squished from the tier above it. All my technical cake talk is so complicated :) Hope you enjoy looking and learned something from my experience. If you have any tips on spheres or fading fondant, please share with us all in the comment section.