No system is perfect, and at some point there will be fraud. From what I recall, at every single point the patient has the ability to stop the process up until the last minute. All patients require to be in a state of mind to keep making authorizations at every step. I understand your concerns, but I would be willing to take this risk, as I really want both my parents and everyone I know to have this option. Whether they choose it or not, I will always respect and support them at every step, but leaving them in agony and continuous suffering is not something I'm willing to overlook.
IIRC the patient can make a request in advance specifically covering the case of them being too out of it to consent. Obviously, in such a case if they're then out of it there's no final authorization needed.
But I genuinely understand your concerns.