Is it possible to have a spinal csf leak without evidence of sagging on brain mri? if so, how do i press issue with md who refuses to chase further despite classic symptoms (postural headache, nausea, lightheadedness)?

Are you stuck with a particular doctor or do you have any freedom to see someone else? Are you able to request a visit with a Neurologist without any further evaluation first? There are many possible explanations for your symptoms, and additional information is needed before making a diagnosis.

Fully agree with Dr Madonna. If you are dis-satisfied with your current doctor, it is advised to seek care elsewhere. A neurologist would be an excellent resource.