For the record I am not medically trained, just a fellow sufferer who’s been self-taught (out of necessity!) for the last 10 years and I have spent an awful lot of time studying Head/Neck MRI’s. These are just my thoughts on the matter:-
Many of your symptoms are typical of this type of instability, between C1 and C2, called atlanto-axial instability, i.e. the dizziness, head pain, neck pain, difficulty swallowing, tinnitus. The medical sign I see suggesting this may be the case, is the ‘V’ sign – between the front of C2 and the back of the anterior arch of C1, as I’ve highlighted in orange on your MRI below. Usually these 2 anatomical parts are quite parallel.
I need to ask a couple of questions:-
1. Are your symptoms worsened by rotational movement of the head ?
2. Are your symptoms worsened by putting your head into a chin-down position (flexion) or after being in a chin-down position?
Your odontoid is not overly retroflexed but, if there is instability in your upper neck, it will possibly be acting as a fulcrum, whenever you put your head into flexion.
Another thought on the matter is that, even though you have been decompressed, your cerebellar tonsils still seem to be clinging on to the back of your brainstem.
Hope some of this helps. I’m glad you’re approaching The Chiari Institute, they will be able to advise you further.