Are you (or anybody reading) aware of any Alzheimer's studies that looked at CSF drainage? I just found this 2017 paper: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5577629/
One thing that to me makes me think it might be more correlation than causation is that the nasal turbinate pathway is not the major CSF absorption pathway. We know this because people who have blocked CSF circulation lower than that, end up with high pressures inside the skull and require an EVD or else they die.
In addition, lumbar punctures collecting CSF for analysis are used for some diagnostic studies. I did not see anyone suggesting that the lumbar puncture itself might help with Alzheimer’s.