When you ice the cake, try paddling it back and forth on the cake if it's developing bubbles as you're icing it. I would avoid the whisk after the butter is added because in my experience that makes it full of bubbles.
I've had that problem with discoloration using a hot scraper, so I don't do that anymore. In the future, switch to the paddle when you add the butter, and if you get air bubbles then gently stir it by hand or smash it against the inside of the bowl to get the bubbles out.
Rachel, if I make a dam with a powdered sugar based icing, I'll make it stiff and pipe it about 1/4" in from the edge of the cake. That way it has room to expand. You might want to look up the thread from leah_s where she talks about using a tile to force settling.