Interesting thread: My husband is a retired airline captain with 38 years experience and has related many post-trip medical emergency stories to me.
First of all,the crew doesn't make medical decisions unless they are very simple problems, such as a passenger who is hyperventilating. The cockpit crew is immediately notified of any serious problem and the cabin crew makes an announcement for any medical personnel to ring their call light. I have done this many times, had a FA quietly come to my seat, I tell them my credentials and experience (for me: RN: Labor and Delivery, OB, peds) and they decide later if I am needed or not.
For serious problems, the flight crew contacts their medical advise folks, Docs at Med-Link I think it is. Those docs will give orders depending on what information is fed to them from those in the back. The captain is called the Pilot in Command for a reason and he or she will make a decision to request a diversion or to press on, with all the information being taken into account.
One on occasion, my husband was mid-Pacific,half-way between the West Coast and Hawaii, when an old man coded. Not knowing at the time that he was DNR and going home to die, the crew started CPR and my husband contacted Med-link. Mind you, this guy was gone and there was 3 hours left in the flight regardless of pressing on or turning back. After talking to his wife, finding out he was DNR. my husband told the crew to dc CPR and do what they could to cover him reseated next to his wife. The wife sat with him holding his hand for the remainder of the flight. Med-link wanted them to continue CPR for the remainder of the flight. The PIC said "that's not happening." Med Link then said to restart CPR on approach (I guess to make a show of having done it according to the book) and again the PIC said, "and that's not going to happen either."
Med-link will be notified for all serious medical problems.
Years ago, before the Med-Link thing, he told me of a situation where 5 docs of different persuasions responded and my husband did a quick interview and he decided which one of them would be in charge.
I told my husband that if I were ever on one of his flights and someone went into labor, I, the experienced L and D nurse, would be in charge. He knew I was serious. I am only about 1/4 joking about that, because unless there's a veterinarian, or an actual midwife or OB also on board, I can guarantee I would have had more recent experience and would have "caught"more babies than anybody else who might show.....
I have made my presence known several times in flight...I have never actually had to help. But I would.
Personally, going through an entire ACLS procedure mid-flight anywhere, without having any kind of facility for transport within 15-30 minutes from door to door seems like overreach to me.