As far as I’m aware NO2 will fill a pit. In the industry I work in NO2 leaks are serious threat because the engines that are worked on are in, what is essentially, a cage that you walk down into. I’ve not experienced it myself but heard from someone who experienced it firsthand as you just start feeling sleepy. I know now that it’s because our bodies are sensitive to asphyxiation from CO2 but not NO2, so as it displaces the oxygen in our lungs you just start getting woozy and tired.

All that being said; I am not an expert. Half this is information from diesel mechanics who work with it and the other half is from random science videos. So take it with a grain of salt.

It’s just heavier??

Most slaughter houses use bolt guns.

It's hard to find official statistics, but most definitely not.

It is clear, however, that in the United States today, “CO2 stunning of pigs is the major method that is used in large slaughter plants.” According to unpublished data from the Pig Improvement Company, the use of CO2 gas to stun pigs has increased dramatically in recent decades. In 1999, CO2 was used to stun 2 percent of all pigs and 2.2 percent of pigs in establishments that slaughtered more than 4,500 pigs per day. By 2020, those numbers had risen to 86.2 percent and 96.2 percent, respectively. Today, according to FSIS enforcement records, at least 32 slaughter plants use CO2 gas slaughter systems.

https://www.fsis.usda.gov/sites/default/files/media_file/documents/23-05-AWI-05162023.pdf

Zero pain, or as minimal amount of pain as possible. Like, microseconds.

If you assume, that the operator doesn't make any mistakes and is always 100% on point. Which they are not, as has been documented by countless of hidden cameras people have put up in slaughterhouses.

And even then, you are ignoring the immense pain and stress the animals experience the rest of their life before getting killed.