First, let me say that I read your entire post.
Second, I get your concerns. I do. But when they found him he was bleeding out badly and was too weak to resist. If they had read him his rights, he'd either be so weak he might not get it; unfamiliar with the process as a non-native to the country; or doped out of his mind and unable to answer. If they read him his rights it will be after he is back down from his drug-induced state (this assumes that he will not simply expire from his wounds in the hospital, which is what I personally hope happens. Yes that would mean they'd get no information from him, but honestly I doubt he'll give anything up anyway when he's a professional killer at age 19)
In this case, also, the public safety exception clearly should apply. The guy was (is) a psychotic bomber, who carried out a plot to kill and maim as many people as he possibly could. He and his dirt bag, now deceased brother only managed to kill four people - including an eight year old boy - but they clearly planned to kill many more based on the suicide vest his corpse of a brother was wearing and the pipe bombs and the like they were hurling at the pursuing officers.
As to your drug mule example: This one I disagree with. The Police usually use the drug mules to get to the traffickers. They usually give them some sort of deal, since in some cases they become drug mules simply because they feel they have no options.
Proud of the Six One Seven.
We are Boston. We are strong. We will prevail.