The virtue advises to never contrive to have it fulfilled in some unexpected way after the fact, you have to give fair warning.
"This is a Major Virtue because the character knows he can get away with insane risks; sneaking his prophecy up on him is an unfair way of negating the value of the Virtue."
They are supposed to be taking insane risks knowing that they won't die without a warning, that's the virtue working as intended, not abuse.
You can have the reason be all sorts of things conceptually related to the prophecy, but you always have to warn them before-hand. The nobleman doctor is an excellent example of dying at the hands of a prince, but you should say something along the lines of "hey, this guy counts for your prophecy, are you going to let him treat you and risk him killing you if he messes up or are you going to try and find someone else and stay injured until that happens?" You could mention it before the fight with the guardsman by saying "the nearest doctor is a nobleman and he counts for your prophecy, I would suggest not getting incapacitated if you don't want to risk that guy killing you when he treats your wounds and you are unable to tell him not to" or something like that.
In my troupe I believe it is if you get take an incapacitating wound outside the parameters of your prophecy then you don't die, you're just incapacitated and roll to heal as normal, except you can't die from rolling a 0 or less. We've not had the Death Prophecy guy get hurt that bad so far, but I think that's how we've said it would work.