I confess that I've not read the entire thread, so someone may just have made this observation already.

Thinking about whether the gas expelled is lighter or heavier than air is only relevant if those respective gasses are at the same temperature and pressure. Or at least the question is in no way decided by the lightness/heaviness of the respective gasses at a standard temperature and pressure.

The within-bowel gas is going to be warmer than the ambient atmosphere of the animal host, and being elastically contained by the gut, this in itself makes for increased pressure. Then the pressure of peristalsis (the propulsive squeezing of the gut) will also raise the pressure, resulting overall in more mass per cubic centimeter of compressed retained gas.

It seems obvious that a fart--especially a good, round, explosive fart, resulting from high pressure--will create a momentary weight loss.

Dave Oesterreich