CO2 has already been proven to be a humane form of euthanasia for mice and rats. Just because there are some YouTube videos showing it improperly done, doesn't mean it's inhumane.

Suffocation is LOSS of airflow. As long as there is a slow introduction of CO2, breathing remains normal. As soon as enough CO2 slowly enters the animals system, they pass out. Then CO2 is increased and the animals brain will die of a lack of oxygen. There are plenty of other videos that simply show them lying down and "falling asleep".