Someone has to clean up incidents such as that. I've known people who worked for companies who were contracted to clean up homicides and suicides (ServiceMaster). They were kids barely out of high school and the experience was traumatizing for them.
A person in these dire circumstances is likely also someone lacking the resources and ability to accomplish any of these "DIY" schemes. People in care homes are there because they have trouble taking care of their basic needs.
I'm pretty sure that's a myth, at least in most areas. Helium tanks usually contain >99% pure helium. I've never seen one with a significant amount of oxygen.