If they control the document, they control the decision. You can’t authorize anybody else to actually make decisions like that. You have to state it as your decision in the document. At least that’s the case in my state. (Mo)
As far as family members capabilities, one simply must ensure that one’s family member (or other trusted person) has had a proper Catholic formation.
I would dispute your assertion that 99.99999% of doctors and nurses are not into “mercy killing”. It depends on what you think of as “mercy killing”. I recently attended a conference presentation to medical students of all kinds, the subject matter of which was precisely the purposes and content of advance directives. I was there at the invitation of a nurse presenter. I really was appalled at how cavalier some of the presenters and a majority of the audience was about refusing care to the elderly or disabled. Maybe they all get a change of heart later in life, but I have my doubts. One of the physician presenters disclosed that his own father had terminal cancer but was not in terrible discomfort or in imminent danger of death from that. He wasn’t comatose normally. His father contracted pneumonia and was unable to voice decisions because of that. The doctor and his father’s doctor consulted together about whether to give his father antibiotics for the pneumonia. They decided against it because, after all, his father was going to die eventually anyway. So, his father died of treatable pneumonia. Why did they do that? Because they decided his father’s “quality of life” wasn’t very good and would get worse because of the cancer.
Most of the attendees (hundreds of them) clearly appeared to approve that decision.
The nurse presenter (a Catholic) opposed that view from the rostrum. After it was over, a handful of students came to her and informed her they felt as she did and thanked her for opposing that doctor’s viewpoint. But it was very clear to her that the majority did not agree with her.