How are you doing, Brian? I am 48-year-old male. This year, I had an MRI that showed a large sub-arachnoid cyst. I didn't have your range of symptoms. But I had continuing headaches, and severe short-term memory loss. The neurologist told me that no treatment was necessary. My wife couldn't believe it, and we immediately sought a second opinion. The next doctor sent me for memory evaluation with the Weschler tests, and it turned out I was far, far below the average score. The doctor recommended surgery with endoscope through a small incision on top of my forehead. In the four to five-hour procedure, they cut a hole in the wall of the cyst ("fenestration") to allow the accumulated cerebral-spinal fluid to disperse, and reduce pressure. About a week post-surgery, the memory tests were repeated, and the memory disability had vanished. My scores were now well above average. Six weeks post-surgery, I feel largely over the problem.
My basic online reading tells me that subarachnoid cysts may be left untreated if they are asymptomatic. Since I was lucid, and not suffering a range of symptoms like you, I guess the first doctor thought I would be OK. That was a pretty bad mistake. I was explained by the second neurologist that my cyst position ("velum interpositum") was unusual, and it was directly bearing on the fornix (which is part of the memory-creation apparatus). Without intervention, I was at risk at permanent neurological damage.
I hope this helps you, or others in this situation.
David