The negation is: “there is an indispensable [crucial] aspect of residency that requires exceptionally long hours.” If so, the author’s premises about other professions wouldn’t support his conclusion about resident physicians. The author must therefore assume that this is true.

The author doesn’t need to assume that this is true: his premise is that physicians have an effect, not necessarily that they have a strong direct effect. Even if they had less of a direct effect than the other two professions, it wouldn’t contradict his reasoning.

This would strengthen the argument, but it isn’t necessary for the author to make such an absolute assumption. For example, perhaps only working more than 70 hours a week decreases work quality. The author could believe that, while sticking to his original reasoning.

The conclusion and support are about professions with life-and-death consequences, so other professions are irrelevant here.

The author’s reasoning is about the consequences of physicians’ work, not their desires. Even if all residents wanted to work long hours, it wouldn’t undermine the author’s argument that such hours are dangerous and should be prohibited.