You should also look up ruptures air sac or Subcutaneous Emphysema and see if that's a possibiility. I'm not saying it is, and to me, it looks like the swellin is in the abdominal cavity, not just under the skin.
Here is one of mine with it:https://www.backyardchickens.com/t/828000/subcutaneous-emphysema-pictures-added
Air sac pictures:


-Kathy
Here is one of mine with it:https://www.backyardchickens.com/t/828000/subcutaneous-emphysema-pictures-added
Air sac pictures:
-Kathy