Hello, I am sorry to hear about your unidentified symptoms. I have read quite a lot about subarachnoid cysts over the past three years, and I had surgery for one in summer 2017. At that time, I was initially told to go home and not worry by the first neurologist. My wife was getting frantic with my memory loss and confusion. She kept pushing and within a couple of weeks, I had a definitive evaluation that I needed immediate surgery. The surgery was highly successful. At about that time, I had developed tinnitus, but the surgeon ruled out any connection between it and the cyst. I have had no problem with smell, and I don't recall anything about it from the reading I have done.

On MRIs, a cyst can appear as a milky white, or dark, blob at the edge or the middle of the brain. They can be pretty large. This is the first image from Wikipedia. https://en.wikipedia.org/wiki/Arachnoid_cyst

My experience seems to be that the location of the cyst can exert specific impacts on neurological function, but often amnesia and dizziness, due to the pressure the cyst exerts on the surrounding tissue. Cysts develop when brain fluid gets trapped inside a normal membrane. As far as I know, these cysts usually do not resolve on their own. The treatment basically involves puncturing the cyst wall. If they are not causing trouble, it seems that surgery may not be necessary (which is why I got the "dont worry" advice from the first neurologist). But if the cyst is in an esssential area they can cause a lot of trouble.
Keep pushing for answers.
Let us know what you find out.