If I wanted to make a dummy, not using fondant, do I just use regular buttercream or royal icing or something separate completely? I know that there were dummies long before there was fondant...or at least before it was commonly used so there has to be a way
Sure you can use buttercream or royal--the fun of using royal is after it sets up you can sand it smooth--same with perma-ice.
I've used buttercream with no problem. You can smooth it like you do on a cake. I tried royal once, but I ended up spending so much time trying to sand it smooth that I gave up and went back to buttercream or fondant.