Hello @chzuck. I believe, like your ENT, that if you use a spray like Neosynephrine for more than a few days then it does trigger a rebound. For me, that effect shows itself about 12-24 hours after last use, with increased congestion again. Since this is the symptom I am treating in the first place, it is hard to know the difference. I use it minimally only when severe congestion from a cold. He needs you off of it to help in your diagnosis. I would try to eliminate the possible allergen effect by doing the super clean of the bedroom with new pillow, clean bedding, mattress cover. Also consider the possibility of an allergen involving the cpap device. I don't know much about them but inadequate cleaning could result in a fungal growth inside.
There are other possible causes for your positional congestion, but you have to eliminate the common ones first. I found that a bedroom deep clean, new pillows, mattress cover all resulted in alleviation of much of my nighttime congestion. Saline nasal flushes and Breathe-right nasal strips help a lot, as well as just good hydration. My cousin was in your situation minus the cpap and after years of symptoms and nasal sprays, both decongestants and steroids, recently moved to a new home. Instantly symptoms are gone. With poor eyesight and limited mobility, her cleaning skills had been zilch.
The medical saying " When you hear hoofbeats, look for horses, not zebras." means attack the frequently seen conditions and if all that proves to be no help, then look for the rare underlying conditions. Good luck working on your horses.
Have you been able to breathe without the Neo-synephrine? Can you use nasal strips with your cpap mask?