Firm palpable lump by spinous process doesn't hurt. Ct scan normal = "normal tissue". Can cause lower front teeth to throb, ache? Even post root canal?

Probably not. A firm lump on the spinous process of a neck vertebrae can be examined by a primary doctor, and then a neurologist, neurosurgeon, or ENT doctors to get ideas on what it is. If the ct finds no problem, the doctors can help decide whether or not more evaluation is needed. That bump is probably not causing toothache, so the question for one's doctors is: is there something else causing toothache?
It is unclear... It is unclear exactly what spinous process it is to which you refer. However, it is usually prudent to explore the more likely culprits in dental pain before broadening the diagnostic search to more exotic suspects. Lower incisor teeth can be deceptively difficult root canal candidates, and it would be prudent to consult with an endodontist to assess the status of the endodontically treated tooth.