Generally, yes, you'd use a board under each tier cut to size. However, some people don't do that, especially if they're using fondant. You may see some people just spread royal icing in between each tier.
Yes. This is me. LOL. But I also just like more cake than filling I guess. I do tend to pile on the buttercream so like AZ said, I like to have a balance with everything. I do like having three layers of cake and two layers of filling, but I don't have three of each pan size so I almost never do cakes like that (only for family).
I don't torte either. I do two layers of cake and a fatter layer of filling. If I had an Agbay I might give it a whirl, but generally speaking I like a bigger ratio of cake to fillings and buttercream.
Yes it would definitely need to be brought up to room temperature before eating. I was assuming that you could get there and set it up an hour or two ahead of time. You don't have to chill it so it's rock hard all the way through, just enough to firm up the buttercream.
Ok, maybe experiment with some powdered sugar based icings first before jumping into a meringue buttercream. The meringue recipes can be tricky for some people starting out. Things have changed quite a bit! Fondant was kind of an exotic thing when I first started a long time ago.
Have you tried a meringue buttercream? I use Warren Brown's recipe which you can find on YouTube. You can also try decreasing the sugar in your recipe if you don't care about crusting. I will usually add lemon juice and salt to my powdered sugar based recipe (I use a couple different recipes) to cut the sweetness. You can experiment with many different recipes if you don't rely on crusting for a smooth finish.
I would take that exact picture and do it in fondant on a round cake. It would be easy to do the balloons, but the dog might take more thought. You could try to find a clip art version and see how he looks in a "cartoon" format. But this is relatively easy to transfer into 2D fondant.