Possible poisoning scenario that maybe hard to detect particularly if the person is
Okay, so Formalin / Formaldehyde in live humans particularly by injection is not researched for likely obvious reasons. There was a case of a Russian woman given a bag of it instead of saline in surgery (she died horribly). And here is a case of inhalation. Injection and absorption has been known to create explosive diarrhea. This is uncommon enough not to be something most would be familiar with and is of course used in autopsies and in embalming. Daybell could have had access to some or known how to acquire it from his job as a sexton.
Imagine you go in for an autopsy the process involves using formalin to fix the lungs. But the lungs are already effected by an fatal dose of formalin. Or you do an autopsy after embalming of someone whose body was had been poisoned with formalin prior to embalming.
These effects are also widely discussed in anti-vax groups, who on a Venn diagram would overlap heavily with new-age ideas AND preppers. So that's three contact points for Daybell being introduced to the effects of formalin on human bodies.
AC was known to be a drug user. His new wife's first reaction was to give him naloxone. Just suggesting a way to get him to inject it. Otherwise CD was right next to TD.
Description of a case that involves foaming at the mouth with formaline.
The Sanitary Record