Are you in the US? I don't get any of this. Not sure anyone on here can read an MRI, but if you don't trust the guy doing the tests, why keep getting tests from the same place? You can get an MRI all over the place if you are in the US at least. I also have no idea why a GP wouldn't have access to an MRI, but most of them wouldn't be able to do much with one as they don't specialize in anything, they are generalists who do a bit of everything. They can read one probably. I do have to say, specialists don't always agree with the conclusions of a radiologist, and if a doc orders an MRI with contrast, that's what you get. So a lot of this is confusing, as radiologists read them and interpret them but don't prescribe them.
I'm not a medical professional, but I have looked at lots of scans of people with CSF leaks. There is no brain sag into the foramen magnum, like you see here: https://spinalcsfleak.org/wp-content/uploads/2020/02/cranial-MRI-pre-post-800x450.png
While your symptoms do sound very much like a CSF leak, upper cervical instability can cause the same symptoms. I would really look into that as a possibility. Atlas orthogonal is an amazingly helpful (and noninvasive) procedure to correct that.
Go to another radiologist.