Another update to help things along. Whatever you do don't use XYZ other than what is here until you understand the implications. It really can do a number on the mesh cross platform.
I modded this a bit for more control.
Also show what just moving the mouth zspheres in and out from the head can do. Please note this is the chain from the head to the upper or lower mouth part.
I've enclosed 2 versions, what I start with and what I use before sculpting - it's important. So says I!