Try painting a thin layer of latex over your armature before you sculpt. Let it dry then put the clay over that. Your armature will pull away from the clay with ease when demolding.
If properly taken care of latex wont deteriorate until long after you're dead... or close to it.
Silicone is great because it has an intrinsic realistic flesh look, movement and feel. If you want to attach it to an under-skull whats the point of realistic movement? Annnd if you're making a...