your last para 'perhaps the solution......' - perhaps also having a fourth POS at the jaw base and the fifth POS placed at the back of the lower neck [not sure if this is safe in the neck area, as I'm still awaiting the Zapper book to arrive!
So in effect one POS on jaw base and one POS rear at the lower neck area might prevent any migration back to the head provided of course they have first migrated down from the head.