Have you ever used a SCUBA mask to take breaths with your mouth. Your literally have to bite on the regulator mouthpiece and deeply pull/inhale the oxygen in. You are right, when you pass out and become unconscious, you do not decide to breathe, breathing is autonomous, it automatically happens.
However, when I took a first aid class, they thought us when someone has fainted and is unresponsive, the first things to check for are breathing and for pulse. The way you check for breathing is put your ear to their nose while looking down towards their chest. You should be able to hear them breathing and feeling the exhaling of air in your ear as well looking to see if their chest/lungs are expanding (moving up and down, you can also place a hand on their chest to help you gauge if their chest is moving up/down).
This video of British Red Cross first aid illustrates what I described with the ear to the nose (@ 00:35 & 1:15) -
One thing you never see an unconscious/unresponsive person do, is attempt deep inhales to attempt to breathe, and especially through the mouth.
My point is, SCUBA mask breathing is not a natural way to breathe. An unconscious person breathes in the most natural way, which is for most people through the nose, and certainly not in a labored pulling (deep inhalation) motion required for a regulator.
You are saying the OceanReef Mask you have chosen for yourself allows for nasal breathing, which I believe should be an important consideration when going with this method. So if Nasal breathing is in an important factor, I don't see how SCUBA is better than SCBA.
All SCBA masks allow for nasal breathing and you don't have to bite down on a mouthpiece to breathe with the mouth as you would with SCUBA equipment, which again you have to question your unconscious body's ability to remain biting down on the mouthpiece.
Can I ask why you consider SCUBA OceanReef Mask as the better equipment for this method over SCBA equipment?
