Hi and welcome to the Chiari forum.
Many times those with Chiari have under lying issues and or conditions that are related to Chiari....if ur Drs r not well experienced with this u can have issues and set backs in recovery....and keep in mind surgery is not a cure,...only a means to restore CSF flow and slow progression.
U mentioned u had 2 surgeries...may I ask what the 2nd one was for, and what was done in each surgery?
Lamenectomy, duraplasty....etc.? cerebral ptosis?
Were u checked for syringomyelia, in the cervical, thoracic and lumbar spine?
Did they rule out disk issues, ICP, POTS, ehlers-danlos?
When you say the tests were last year when last year? I'm in the UK and only know from having a wife with the condition what our experiences have been to date. I don't know if your neurologist is a local neurologist at a local hospital or a state specialist dealing with main individuals with Arnold Chiari; however I would recommend seeing the most highly specialised neurologist you can. I can particularly feel for you and understand the concern about the swelling at the back of your neck too and I would certainly be asking for new MRi scans to be done and even a second opinion from the highest specialised neurologist I could find. Unfortunately though so little is known about the condition that you may well be at the front end of the knowledge of this condition, in which case I'm unsure what else to recommend.
Out of interest have you tried removing all types of sweeteners (apart from sugar itself) from your diet? Sweeteners such as aspharame and many others have been known to create MS, ME and other neurological confitions worse or even immitate them. My wife has been helped by getting off all sweeteners. She also has less symptoms when she has a vitamain complex. I think that imbalances is B vitamains may be the reason but the complex seems to help alot.
I hope this reply is helpful to you.