Select the joint where the head is, then hold down the shift key and press "p".

That will unparent the joint at the head.

Then you can put one or more joints between them.


Now, when they are in place, select the neck joint you added first, then the root joint(s), and hit the "p" key to parent them to the root joint.

Select the head joint, then the joint you put in and hit "p" again.

The same process will fix the jaw problem.