Hi the key is getting your point order correct. If you make your blendshape you can uncheck the check topoligy option and then you will be able to have a blendshape of only the head working on the whole body. So this is what I do:
1: Seperate the character mesh into 2 parts the head and the rest. ( can be more but this you can figure out later)
2: select first the head then the rest and combine the mesh and reveld the verts and uvs.
The point order wil now run from 0 to whatever the amount of verts in the head part and the body wil be the rest.
3: Duplicate the mesh and delete the faces that is not part of the head cut you made earlier
This is now your blendshape and can be duplicated for all the blendshapes you want to make. I usualy make one that is effected by all the other and then only that one effects the character mesh but that is only my preferance and not needed.