This idea is rather redundant, as there is a mod that allows you to be whatever mob you want, as long as you successfully kill that mob. It is called the morph mod, and it can be found here: http://ichun.us/mods/morph/
If by shape shifter mod you meant this one, then how didnt it work?
Perhaps, but I bet that if someone could figure out how mods such as morph and custom player models work, he could just make the model change and then give the player a special spell to shoot wither heads and so forth, and eventually it wouldn't be too hard to make.
That's editing base code, however... not a good idea. So many things can go wrong.