By ‘instability’, I mean that the ligaments are not doing their job properly, i.e. they are not maintaining the strength of the structure. The ligaments should hold all the bones etc together, so that it moves as one unit.
I actually have cranio-cervical instability, where my head is not firmly connected to my neck, following a backwards fall years ago so, experiencing many of the symptoms that you do, rang a bell for me. I have to wear a philadelphia collar 24/7 (the conservative treatment!) to avoid or lessen some of the symptoms.
You may have had instability prior to your operation. Instability after decompresion can cause intermittent symptoms. Decompressions often address the immediate issue of ‘restriction of csf flow’ but unless combined with fixation, they neglect to address, or can even worsen any instability.
It’s often the case that whilst you are laid down, with your neck comfortably supported in an MRI, no signs of intermittent csf restriction would be visible. I think a good test would be a CSF ‘cine flow study’ MRI, with the head in flexion.
I think people are probably born with a retroflexed odontoid (or maybe the result of injury) I’m not sure. It’s just an anatomical difference. In a stable cranio-cervical junction, I can’t see that it would pose too much of a problem.