after some playing with blendshapes, keeping in mind the skeleton is ik-rigged, i came to this conclusion.

creating blendshapes with IK is really bad!

even if IKblend is off, all joints inherit funky rotation values in all axes. therefor there´s no clean way to link joint.rotation to a blendshape, for the very reason of the foot´s IK which is sticky and while bending the leg the foot stays on the ground which is resulting in geometry deformation - ergo it´s making no sense duplicating the geometry for using it as a blendshape.

long talk short sense -

would anybody agree to create blendshapes before setting up the IKs?