I have always transported my cakes assembled because I always want to have enough time for decoration and assembly without feeling rushed.
1. If you have TCB there is a master chart for frosting quantities. It will depend on if you have layers to be filled, etc. ALWAYS make more than needed - it can be frozen with no issue.
2. To stack the cake I use bubble tea straws. Agreed with prettycake, about 4 or 5. If assembling prior, you can also pound a wooden dowel through both cakes. I usually sharpen it with a pencil sharpener and I also make a hole in the centre of the smaller cake board so I don’t stress the cake board. Make it about the size of a quarter?? The dowel should be shorter than the height of both cakes. You will need to fill and smooth the hole after. I have also done this the “reverse way”—where I set the top cake over the dowel so the surface isn’t marred. In this case, I make the hole in the board a little bit bigger.
I usually clear out my fridge and find a clear storage bin that will fit the entire assembled cake with display board/platter. I assume you will place the cake on a display board/platter—basically something strong that the cake will be displayed on. I attach the cake to it with some frosting (basically put some frosting on the platter, place the larger tier on this 1st, then do the straws and add second tier and then the decorating/edges/borders). I use the lid of the container as the bottom and set foamy shelf liner on it to prevent slipping. I set the completed cake on the lid and then place the container over the cake. I do this because it is much easier to lift off the container than it is to pull the cake out of the container and the container is usually wider at the top (which is near the bottom of the cake when the container is inverted). BUT you mush be careful to ALWAYS pick up from the bottom—so don’t allow anyone else to pick up your container—put a sign on it if you must! Also, I use a clear container because I can see the cake when the lid removed/installed and because they usually don’t have the odor that some colored plastics have.
I would recommend refrigeration WITH a container. Otherwise, the cake could be subject to: a) odour, b) debris/dusties/fingers/etc. that may inadvertently touch the cake, c) possibly drying the cake, d) condensation. If you pull the cake uncovered from the fridge, condensation will take place immediately—as the warm, humid air condenses on the cold cake. If you remove the cake in a sealed container, the warm humid air can’t condense on the cake. There is often a small amount of condensation even when the cake is covered, but it will be less than if you remove the cake straight from the fridge, uncovered. I have had nasty condensation issues…so I can attest to this!!
Good luck! Wedding cakes can be so much fun, but they are also stressful! Can’t wait to see your results.