Very common for memory loss to be associated with Sleep Apnea. Memory should return with treatment. As for the increased sensitivity to smell?
I think your doctor was correct to rule out NPH and the spinal tap, but NPH can be very difficult to diagnose because it mimics other disorders such as early stages of Alzheimer's. Enlarged ventricles can be seen with MRI, that is if you have a image 1-2 years earlier that they can compare it to, very few people have that.
As you know CSF is mainly flushed out with the blood and new CSF is produced to replace it. All part of health.
In order for that blood exchange to take place you have to be well hydrated. As we get older our bodies don't do such a great job at letting us know we are thirsty, so you end up dehydrated. So you have to make a conscience effort to replenish your fluids.
I'm only 50, I tried once to drink the recommend water per day (8ea x 8oz. glasses of water or 64 ounces per day), it meant I nearly had to carry a water bottle around with me all day taking sips every few minutes to drink that much. I think if you wrote down actually how much water you drink, you have a hard time drinking half that. Nothing works when you are dehydrated.
I would also suggest a good multi-vitamin like Centrum Silver for adults, it seems to be well balanced and contain the vitamins that improve cerebral blood flow which helps with memory. Those vitamins are mainly B6, B12 and Folic Acid (Folate). The one I find is pretty good is Costco's Kirkland brand Daily Multi for Mature Adults, it is very easy on the stomach (I have history of ulcers), it is well balanced and probably has everything you need. It is item #928446.
Drink lots of water, make sure it is pure not that city tap water full of nitrates, chlorine, fluoride and Chevron ethyl