I don't believe they are caused by the brain or body because many people travel in spirit to love ones further away during an NDE and what they reported seeing that person doing ends up being correct. Or like one example, a guy floated above and saw a shoe on the roof that you can't see from any other angle. After his NDE he checked and there was indeed a shoe on the roof where he saw it.
I think an NDE can be planed before birth, but I don't think all of them are. I know one kid that drank so much alcohol that they had to revive him. I don't think that was planed before birth.
I think they can be healthy in some cases, but people can also get the wrong idea about the experience. Like some of the Christians who have had an NDE and had an experience of hell come back and warn everybody about how bad hell is and put the fear into people. Then their message becomes "fear is a real place and you better straighten up or you're going to end up there".
There is more than one book like that out there. What they would have discovered if they had progressed to a different stage of the NDE is that they would have likely left the hell planes. They put themselves in the hell experience, and they would have likely left it after asking for help or after they came to realize they were doing it to themselves.
I recommend the book "Blessing in Disguise: Another Side of the Near Death Experience" by Barbara Rommer. She categories less than positive near death experiences by stages. Apparently we go through different stages during these experiences. It sounds to me like those who only experienced hell didn't progress through all the stages. So they don't have a complete picture.