Yes, "when they cannot reasonably be expected to prolong life" is key, since there's some discussion in other parts of the thread about withholding nutrition as a form of euthanasia, which Catholic teaching is very much not okay with.
Not as much as you'd think. Catholicism is very much has what is essentially statutory law and common law, and common law removes many of the ambiguities. It's the Protestants who are like "we all have the same book let's talk about how we interpret it" while Catholics have a canon interpretation over which much blood has been spilled to arrive at.
From the url: When inevitable death is imminent in spite of the means used, it is permitted in conscience to take the decision to refuse forms of treatment that would only secure a precarious and burdensome prolongation of life, so long as the normal care due to the sick person in similar cases is not interrupted. In such circumstances the doctor has no reason to reproach himself with failing to help the person in danger.
