Can the bag over head method (like helium) not be used with nitrogen?
~Any inert gas (helium, nitrogen, argon) will be nearly equivalent.
"Inert" literally means it ~doesn't "do" any chemical reactions at all. Really,
all the inert gasses are doing is to "displace/replace" ~all oxygen and "push/purge out" any carbon dioxide (respiration's "waste product").
Our bodies are
very sensitive to a buildup of CO2, but ironically have no "sensors" to tell us oxygen is absent (in nature/evolution, suffocation/drowning/etc. obviously a buildup of CO2 was "selected" as the signal to us that we were "in trouble").
So, the inert gas has 2 roles: Replace the oxygen and push-away any CO2.
The inert gas itself isn't poisonous per se... none of them do anything to you chemically.
Actually, air you're breathing
right now is ~78% nitrogen. Nitrogen tends to be cheaper to buy in bulk. I have a large nitrogen cylinder.
Why do you require a mask and regulator?
It's a preference thing: The "exit bag" is cheaper and very well proven... Also, one needs to make sure a mask is well secured (as one's facial muscles tend to relax as consciousness is lost... and leaks right near one's mouth/nose could cause oxygen to get in and interfere).
I, myself, have a bag (hood) setup, but I understand why some people prefer the mask. The PPeH specifically recommends not to use a mask, but if done properly it's not a problem.