MB3 164b: Evaluation of Illness on Shabbos (328:9-12)

What is assumed to be a life-threatening illness? If a physician says it's life-threatening, we obey him. What if a patient says he needs a medicine but a doctor says he doesn't?

Beginning an eight-day treatment regiment on Shabbos (which necessitates violating two Shabbatot instead of one).

סכנת אבר

If Shabbos has to be violated for a sick person who should perform the melacha, a Jew or a gentile?