I want the take a slice out and put on top of the cake. What is the best method for doing this, using all (or mostly all) BC???

I would just cut the slice of cake out and then put a cake board under it. Put buttercream all over it and then decorate it. Then I would lift it up onto the rest of the cake with a spatula.

Although it's just a slice of cake, I'll suggest to place it over a board cut in same sape and support it with a bubble straw before you place it on the cake.