Am I just using the half ball to form the shape until the fon/gum dries then remove (how do I attach to cake) or am I just covering the ball and placing on cake (with fon/gum bottom)?

For all you who have done this, please share your tips, thanks.

I would just bake a half cake and cover in fondant..The gumpaste shell will be too unstable.That way you can poke 2 ears into the cake....I just did a mickey mouse cake this weekend like this and it was very easy and cute.

I used a half styrofoam ball for a minnie mouse cake. I covered it in fondant/gumpaste mix (top and bottom) and then placed the entire thing on the cake. I made the ears from fondant/gumpaste, and then used thinned fondant glue to stick them to the head.
Just be sure to tell whoever is serving/eating the cake that the head/ears are not edible!

I used a very thin plastic bowl for mine and I left the hat on it when I put it on the cake (the cake was for my party). I had thought about removing it, but the fondant would have ripped. I attached my ears with candy melts and it worked just fine. You can see the pic in my gallery.

I wouldn't try to remove it off the styrofoam. I think it will rip or break.