It sounds like you have been dealing with alot recently. Your case is very complex and it sounds like you have had a thorough evaluation. Have you had flexion and extension x-rays completed? This is a way to test the stability of your spine while doing normal movements. It is well known that cervical spine disease can cause headache, balance prblems, and neck pain. Also, even without significant cervical spine disease if the muscles of the neck are to tense these symtpoms may develop.
I would recommend that you be evaluated at an academic headache center. There you can have your films reviewed and a determination regarding further testing can be made. Also they can recommend a treatment strategy to help with your chronic pain. It may take a while, but if there is no underlying cause typically over time with meidcation help the pain will resolve. Good luck.