If the human atlanto-axial joint is pivotal only, what accounts for the craniums additional degrees of freedom?
The additional degrees of freedom are provided by the entire cervical spine, rather than the atlantoaxial joint alone, which is the joint between C1 and C2. What you're describing as roll (when you point one ear to the ground), is provided by contraction of the sternocleidomastoid. The sternocleidomastoid muscle pulls the mastoid process (behind the ear) down and in, toward the fixed clavicle and sternum.  The joint that provides lateral flexion in response is the entire cervical spine. 

Here you can see the sternocleidomastoid muscle (the SCM):



And here, a drawing with a reasonable approximation of the way the cervical spine responds to lateral flexion (due to contraction of the SCM). I can't find a good c-spine film at the moment to demonstrate it in a human person, rather than an artists imagination. But this drawing is accurate to my recollection. You can see the space between C1 and C2 on the right is quite limited (that atlas and the axis are in close apposition). C2 and C3, as well as the other cervical vertebrae, have more space between them on the right: