[I also had an MRI of my head and neck done, and they were both fine. ]

When was Mri done? recently or yrs ago?
things can be missed, if they were focused on one thing and maybe something else is the problem.
Did you request a copy of the MRI report?
It is nice to have that even tho dr says all is fine..
At a certain age most of us have some minor things showing on MRI...

Did chiro have Xrays done before working on you?
does chiro do soft tissue work or only adjustments?
strained muscles that are not treated will pull and affect any adjustments..
my DC used ultrasound, If stim, and low level laser along with adjusting..

Your symptoms sound like more than concussion related only.
Could be something else also..