1) Would having chronically stuffy sinuses (for example due to allergies) theoretically increase reflux to the brain, and/or in some other way increase the negative effects of CCSVI?
it is important that there be no confusion about the abnormal sinuses associated with ccsvi. the veins of the brain drain into DURAL sinuses. these are like blood vessels and are distinctly unrelated to the PARANASAL sinuses which are air chambers in your skull.
however sneezing could increase intravascular pressure that might result in reflux