Right; it depends on the circumstances. There are things you can do to a pig that make them unavailable for other uses, but I can say with some authority that there is medical research you can do on a pig that leaves it available. Also, the pigs the boy was working with hadn't been euthanized yet (avoiding euthanizing them was the point of finding someone to take them).

Well, if they were "euthanized" with narcotics, there are good reasons not to eat them... Or even to give them out to the public for drug addicts to get at.

I think an effectively anesthetic dose of narcotics would be low enough, once distributed through the body, that it wouldn't be major issue. Liver, kidneys and GI might have higher concentrations, but I'd imagine pork shoulder would have significantly below 1/100th of a recreational dose per serving size.

Nitrous oxide asphyxiation could be quite pleasant for the pig, and there would be little risk to anyone eating the tissue.