whipped cream was marginally

When working with fondant/gumpaste - don't wear dark clothes. White white white clothes. There are little bits of lint that rub off of your clothes, especially blue jeans, and these will show up in your fondant/gumpaste. Ew.
I used this mat while working with the fondant covering the cake, and it saved the cake. I had tried rolling the fondant up on my rolling pin a couple times, but the cake was just too large and subsequently my sheet of fondant - it kept tearing as I was draping it. Instead I rolled it out on this mat then rolled up the mat with the fondant, then unrolled over the cake. I was able to get a super-thin (1/8") fondant sheet that way without any ripping.
1/4-1/3" frosting between layers is plenty. I put more (3/4"ish), which accounts for the slight bulging on the sides that you can see in the picture - the weight of the cake was slowly pushing out the frosting.
LEVEL YOUR CAKES. I didn't do this, and had to do some tricksy last minute engineering to be able to stack them on the day of the party. Save yourself the sleeplessness.
Make your gumpaste figures in advance. Way way way in advance. You're not going to eat them, and they don't go bad anyway as long as you don't refrigerate them, and they take a long time to dry. Weeks, it turns out. I made mine 3 days before the party and ended up doing some hi-jinks with a hairdryer to get them to hold together.
Marshmallow fondant is much tastier than store bought, even Satin Ice. Easy enough to make too, save yourself the money (store bought is crazy expensive).
There's some debate on the internet around whether you can refrigerate fondant. I did since my frosting had dairy in it, and I only had marginal problems (get to that in a second). Overall I would say you can - I didn't have any color blending like the 'net threatened. My fondant did start to melt from the inside though, but I think that was due to the type of frosting I used. The whipped cream-cream cheese frosting was absolutely delicious, amazing stuff, but if covering the cake with fondant I would find a sturdier frosting in the future. I believe the whipped cream was marginally too liquid for the fondant to withstand, as it started slowly leaking from underneath starting the day before the party (also causing me to lose sleep).