Relating to the last 5-6 comments....
1. Yes, you have to be a nitrox instructor to lead the AOW nitrox dive. That does not mean it isn't an allowed AOW dive. (It is possible you and I are misunderstanding each other and talking past each other.)
2. Most people consider oxygen and nitrogen to be equally narcotic, so on that theory there is no benefit to nitrox for narcosis. The big question, though, is the fact that some of the oxygen is metabolized, and it is very difficult to test for that effect. A study a few years ago suggested (note the word) a small benefit to nitrox in terms of narcosis.
3. I have never heard of people blacking out from narcosis at 27 meters. There are cases of people blacking out while breathing air at much greater depths, like deeper than 90 meters. Brett Gilliam told me that Sheck Exley sometimes blacked out on air at around 120 meters. April Cohen in Cozumel either was so narced at the planned 90 meters that she continued to 120, or she passed out at 90 and fell to 120--different people give different explanations. The big question is what caused the blackout. It could have been extreme narcosis, or it could have been an effect of oxygen toxicity. If it was oxygen toxicity, diving on nitrox would definitely not be a benefit.
The largest percentage of factors involved in diving fatalities is a medical event, usually involving the heart. That includes sudden cardiac death, in which the individual is alive and doing fine one second and dead the next. In such cases, yes, the individual will pass out at 27 meters--or any depth whatsoever. Or on the boat. That has nothing to do with narcosis, though, and nitrox will not help.