Here's the recipe I ended up using. It was perfect! It works up very similar to buttercream, just a bit softer. Tastes good, too...sort of marshmallow-y. I love marshmallows, but even DH, who hates them, liked the frosting.
We're in the process of trialing dairy (so far so good!) and I made a buttercake with real butter. I substituted rice milk + lemon juice for the buttermilk the recipe called for, though. I would guess that coconut oil would substitute well for butter in a buttercake recipe. The cake I made had 5 layers so it needed a firm cake!
I didn't come up with the castle idea on my own. It's from a Family Fun Birthday Cake book that we checked out from the library. I let DD pick out which cake she wanted. I did scale down their formula, though...theirs called for 4 layers for the main part of the castle and 3 for the tower. I did 3 and 2. And it was still huge!