Tried to switch from FFM to Nasal mask last night to ward off bad air swallowing causing GI issues, and I had success. However, I also kept waking up because my mouth falls open and air shoots out of it, defeating the purpose of therapy. I just can't win

I've seen most people say chin straps do not work for them, and I would agree that it probably wouldn't work for me either. Most that I've read swear by taping the mouth shut, but doctors caution against it, using the reason that if you have a power outage and don't wake up, the buildup of carbon dioxide in your mask (due to reduced speed of air leaving the vent), could cause you to suffocate or fall into coma, since your mouth is taped shut.
My thing is this though: If that is a legit concern, how is that any different than wearing a FFM and having a power outage? If your mouth and nose are inside the mask and you have a power outage, don't you run that same carbon dioxide risk?
So then I considered ordering these, because they say there is an emergency mouth vent, but haven't done it yet because wouldn't an emergency mouth vent defeat the purpose of taping?
https://www.amazon.com/Sleep-Strips-Som ... ooghydr-20