How long do you think it take to loose consciousness with the described method? And how long does it take to die?
As I can see your concern is that the person may grab or rupture the bag while being semi-unconscious. How about laying down on a soft surface (a soft carpet or a bed would do) to stay away from any object that could accidentally leak the bag and wearing a winter coat and pulling your hands inside, so you can't reach for the bag. My guess is that once being unconscious it'd be impossible to put your hands back into the sleeves and reach for the bag.
Time until unconscious depends on a lot. On the Inert Gas Megathread there was a poster (can't remember his name off the top of my head) who tried a few times.
In theory you should lose consciousness in a few deep breaths, (and death following a few minutes after) but the poster I mentioned above said when he tried he has a hard time breathing deeply (due to anxiety if I remember correctly) so it would have taken him longer to pass out. Some members have suggested setting a timer for 60-90 seconds - if you're not out by then you're doing something wrong.
That megathread is really long, but it is definitely worth reading from page 1 to see the give and take between many users, at around page 20 it gets to just be repeated information.
As far as the issue with grabbing at the bag, it is my understanding that if there is no CO2 build up your brain has no idea it is dying, so you will not unconsciously reach for the bag on your head
With that said, I think this is where body position comes into play as well ( but I could be wrong ).
My understanding is as long as there is a constant flow of inert gas the CO2 will be flushed out of the bottom of the bag, and I think all that really matters is your head is upright in some way - I would think laying on a pillow that is propped up some would be okay - something like this posture. You'd also probably want to secure the air hose towards the top of the bag so flow of the gas naturally expels out of the bottom
It is also my understanding that if the flow of inert gas is correct there should be absolutely no feelings of shortness of breath or suffocation. If you feel like you're choking then there is CO2 building up.
Obviously this is all in theory, in the moment I'm sure things are different - psychologically atleast.