Combing methods leads to complications and a higher chance of failure. Pick one methods and plam it well.
Traditionally, suffocation is closing off the trachea so you can not get oxygen in. How would you then inhale NO2? If you are suffocating with a different gas, what then is the point of NO2?
How would you combine inhaling gas from a balloon with being under water?
Sleeping pills will make you drowsey and may help to combat SI but you might also fall asleep before executing your method.
A more concrete plan from you would be better if you want real feedback.