if I use this filling in a cake, how long will it stay firm, at room temperature, without going thin and ruining the cake? I also need to know if I can ice and decorate the sides and top of the cake with buttercream with this filling.
No more than 30 minutes --- generally when you use whipped cream as a filling frosting, you take the cake out of the fridge right before serving. Whipped cream breaks down very quickly unless you add a stabilizer (Dr. Oetker makes one) but even then if the room temp if over 68F you'll lose it in under an hour.
You can put buttercream around the outside of the cake. I've seen it, I don't like it though...too many conflicting things going on.
Adrienne