Perniciuos (Latin for death becasue it was at one time fata) is the inability to digest B-12.
When you don't have enough B-12 your body cannot produce new red blood cells in the quanitiy you need and the red blood cells you do have are enlarged which reduces your blood oxygen.
Pernicious Anemia has pretty much the same symptoms as Sleep Deprivation with the nice extra one of causing permanent damage to your Central Nervous System if you have it for too long.
A blood test will check and see if you have it. If you do you'll have to inject yourself with B-12 for the rest of your life. Don't worry if needles bother you they couldn't make them much smaller than the ones you use for B-12.
Don't trust your Sleep Docotors. Get a copy of your sleep test results and make sure that you are spening about 30% of the night in REM sleep and a large amount in Deep sleep. During REM sleep vital bio-chemical reations occur and it is the rest for your brain. Deep Sleep is the portion of sleep where your body rests.
I can't speak to the folic acid topic. But about sleep apnea - it can throw so many of your body's processes out of kilter due to the stress hormones, and oxygen and sleep deprivation.
Since the doctors have not yet found a cause for some of your symptoms, it may be that getting on track with cpap treatment will be the key to you and your doctors getting some clarity on your case. As symptoms related to OSA begin to resolve, the residual symptoms may make more sense on their own. I'd like to encourage you to take the OSA diagnosis very seriously. I went untreated for many years and paid a high price with hypertension, weight gain, diabetes, GERD, enlarged heart, memory loss and cognitive impairment, and multiple microvascular ischemic lesions on my brain. With early treatment you can regain your vitality and live a life free of the health problems common to those of us diagnosed later in life. Best wishes as you move forward from here.