Welcome to the site, I’m sure you’ll appreciate many of the topics talked about on here. I too was triggered by a head and neck injury, it will be 11 years ago in June. Mine was not a motor vehicle accident however but it was diagnosed (at one point) as Whiplash. I eventually travelled from the UK to USA, in 2008, for intensive investigations which culminated in a diagnosis of Cranio-Cervical Instability (CCI) plus a host of other things!
The head is held on solely by ligaments, so if these get injured (or become defective, as is possible in people with Hypermobility or stretchy EDS) you can end up with a cascade of weird and wonderful (NOT!) symptoms.