Hello,
If you use ultra pasteurized double cream, The ganache can sit at room temperature for a couple of days.
Not knowing what’s in the buttercream recipe I can’t see whether it needs refrigeration or not,
Marshmallows can be refrigerated. Commercially produced marshmallows are coated with cornstarch, so will stay dry.
Fondant does not refrigerate well because of the condensation can degrade it. The condensation can also cause the color to run. But that doesn’t mean you cannot refrigerate it. Place the cake in a heavy cardboard box. Tape the seam to guard against air getting inside. Several hours before serving remove the cardboard box from the refrigerator and leave it on the counter. Do not open the box. Let the cake come to room temperature in the box.
If you do not have a new clean heavy cardboard box, a heavy pot can be used. I’ve never tried this technique but the video link below explains how to do it with a pot.
Cake itself should be baked at least two days before decorating. The cake layers should sit for 24 hours before you begin to decorate. Then once the cake layers are filled, and the buttercream applied, BUT NOT YET COVERED IN FONDANT, the cake needs to be degassed. That takes another day. If you cover the cake with fondant without degassing the cake, air bubbles will form under the fondant.
Preventing condensation on a fondant cake
https://www.yenersway.com/tutorials...ls/prevent-condensation-fondant-coated-cakes/