Presumably people who are on the verge of death and beyond any kind of proven treatments. They're technically alive but have a very short predicted lifespan.
You can try a novel treatment on those but at the same time are limited by ethical concerns regarding pain and future survival (if the transplant "works", you are now in a tricky situation, as you can't easily do anything that has the potential to make the situation worse... and given it's uncharted territory, anything has the potential to make it worse).
Brain-dead people don't have such limitations. You don't have to worry about causing pain nor shortening potential survival, so you can try things that are likely to "kill" them (cause the transplant to fail, or other issues) and learn from the outcomes.
Agree with you on that. People can say that there is a procedure, but verifying that they are following it is another matter. Also curious as to the process for volunteering people for such experiments. Like if there is some paperwork that the family signs off on.
Very good point - I don't have an answer for that. But I'd say if there was a choice between killing a person who is very much alive and conscious by our current standards, and one who is "brain dead" by the same standards, I'd still pick the latter.
Part of it is a choice you make when choosing to donate your body for research. There's a chance brain death can be determined incorrectly (though in this situation it's likely the same determination will be used to withdraw life-support, research donation or not).
If I'm ever declared brain dead, I want scientists experimenting on me. That's a much better use than giving organs to just a handful of people. It pushes the salients forward for everyone.