Pseudotumor Cerebri Support Group
Pseudotumor cerebri (PTC) is a neurological disorder that is characterized by increased intracranial pressure, in the absence of a tumor or other diseases affecting the brain or its lining. Diagnosis requires brain scans and lumbar puncture. Characteristic symptoms are headache, transient visual obscurations or transient visual loss in one or both eyes usually lasting...

sleep apnea increases NO production.
in the presence of sleep apnea, anemia will significantly worsen the oxygen drops at night and the physiological consequences of this including increases in ICP. so if your sleep has improved, it's because your sleep apnea has improved. if your headaches are better it's because your ICP doesn't spike as high, due to improvements in oxygenation during apneas.
stress worsens OSA in a vicious cycle, so by reducing stress, you can reduce airway collapse. hypoxia is one of the worst stresses to the body, really a pre-morbid condition that the body will send up all sorts of physiological alarms for.
it's wonderful that you have made such strides and seen these improvements in your condition by taking D and Iron. and everyone should be doing this. but don't make the mistake of believing the sleep study, which is clearly in error. if there was nothing wrong with your sleep airway, then the anemia would not have made you that sick as a result of sleeping. sleep apnea gets worse as you age, which means that eventually this will come back to roost and then the iron and the D won't help as much and you will need to deal with it in a more definitive way.
Paradoxically, several years ago, before I had done hardly any reading about the NO association, I used to "sun myself" in the spring and summer (mornings) for that extra boost of Vitamin D. I suffered worsened headaches i the spring when doing this since I now realize that the level of melanin has to build up some for a number of reasons. However, the headaches and associated symptoms were worse. This apparently happened due to the nature of NO synthesis in the skin in relation to sunshine, a mixed bag for me. This was made worse by the onslaught of allergens in the air giving rise to a histamine reaction, and histamine can also raise NO levels. Whatamess. Thus, for the upcoming warmer spring days, I will build up to the sunshine in increments and just bask it it for about 10 minutes. Also, I am now aware of the noon day type rays and how they can affect me.
Sometimes, when I awake from a "funny neck position" usually due to pillow alignment, I find I feel crummier for the day due to what I imagine to be the apnea-like situation due to the physics of my neck/airflow, and perhaps Sophiasmom can comment on this occasional apnea situation.
I did find an organic wedge pillow, which I like very much, however, on some rare nights my neck gets "kinked" by some funky position related to sliding off my top pillow (regular shaped talalay pillow) or something else of that nature.
I think that the apnea is always there. it's just worse at times. if you study comparative anatomy, as detailed in Davidson's paper A Great Leap Forward from 2003, this will become clear.