The way the exit bag works is by taking in pure inert gas and breathing out inert gas until you die. If you try to fill a room, you won't be breathing pure inert gas, you'd be breathing a mixture and that would probably only make you light headed and sick. Plus you'd need a ton of helium tanks.
I haven't used a helium tank so I don't know if you can leave valve open, but in US I've heard that commonly available helium tanks have 20% oxygen anyway, so the nitrogen or argon with regulators would be better.
CO poisoning might work in a small bathroom but you may have smoke detector that has CO alarm.
I don't think there's an easy way to avoid exit bag if you want to use inert gas asphyxiation.