I think, nobody can offer you fully evidence-based answer for this, because it would require conducting real experiments with such a drowning method on a large amount of volunteers, which I'm pretty sure have not been done by any research group.
Even medical experts dealing with N2O on daily basis won't be able to offer you more than conjectures based on their gut feeling in regard to your questions. The days when N2O was administered with low % of O2 to patients are gone many years ago. In nowadays practice, giving nearly pure nitrous to someone by a medical professional would be a big fuckup and an extraordinary case, so modern doctors don't really have an opportunity to witness the reactions of people under such sedation.
Maybe if you revived good old sir Frederic Hewitt, he could give you some valuable insights on the given subject, since he observed the effects from administering 90 - 99% N2O many times. I doubt that currently living specialists can tell you more about the corresponding state of hypoxia+anesthesia than you could find out without their help, not to mention that the chances of meeting good and unbiased specialists in N2O anesthesia on this forum are near-zero.
In order to get an idea of how N2O with little proportions of oxygen works on people in general, I highly recommend to read these books
"Anaesthetics and their administration" by F. Hewitt (pages 17, 52 - 56, 76 - 80, chapters X, XIX)
"The administration of nitrous oxide and oxygen for dental operations" by F. Hewitt (chapters IV - VII).
(I wanted to attach these books here, but the forum is glitching again and doesn't let me doing so; however, you can find them in free access on the internet).
Besides that, your personal experiences of inhaling N2O to the point of near-unconsciousness and to the point of full blackout would be somewhat irreplaceable. They could give you good understanding of the shades of semi-consciousness you can have between full consciousness and full unconsciousness, that may help you estimate the likelihood of "waking up" into full or some partial consciousness without access to fresh air.
If you want to CTB in peace, you have to just accept possible troubles, thinking optimistically that you can get over them. Pessimism and anxious attitude only make your situation worse. I started to experience my best euphoric feelings from inhaling N2O only when I completely relaxed and stopped worrying about what can happen next after doing this. It's very likely that optimistic or pessimistic view can shape your reactions on semi-conscious drowning a lot, should it happen.
