Hey blackace
It's a nice hypothetical that you've put down there.
In emergencies and evacuations in partcicular, it's always the greatest good for the greatest number.
"Its understood that most evacs are safe and there is no real immediate danger, only a few cases are really important get out now or die situations."
Any full scale evacuation of an aircraft is considered a "get out now or die" situation; the decision to evacuate an aircraft is not taken lightly as there is ingerrent risk of serious injury to passengers in both the cabin itself and from descending the slides (indeed, there has been a case of a person involved in a full scale test evac to gain an aircraft it's all-clear for service being paralysed).
The bottom line is that in an evacuation, the Cabin Crew's first priority is to get each and every person off of the aircraft.
Generally speaking a medic (not necessarily a doctor, but more often someone who has trained as a nurse before specialising in accompanying ill passengers on fights) does have to travel with passengers such as the one you described, and most (if not all) UK carriers require that any passenger who is of reduced mobility (be the physically or mentally handicapped, blind, or is in a situation that renders them incapable of properly caring for themselves) has to be accompanied on the flight (usually their spouce or family member). This is not only to tend to their needs during a normal flight, but to assist them in an evacuation.
As for who makes the dicision as to whether the situation is serious enough that they should be moved, the initiation of an evacuation is that decision. I doubt that a medic accompanying them would try to override the Cabin Crew in making sure their charge was evacuated, but in the hypothetical situation you have descibed, I personally would do my job and maintain my authority on the situation which dictates that the CC are in charge on the aircraft.
There are no real special ways to evacuate a disabled passenger - they have to go down the slides the same as everyone else, but as they tend to be among the last ones to be evacuated, more care can be taken on how they board the slides.
As for your initial point that it could be more hazardous to move the patient you described than to leave him, the fact is that in an emergency we don't have the luxury of hindsight. It's a simple case of everyone gets out.