I can tell you from personal experience that most people dying from cancer reach a state where they can't eat and are not hungry either. Sometimes this is a result of Cachexia ("wasting" disease). Doctors tend to be overly optomistic - I would take the people in hospice advice more seriously. However I would call them on the pain management issue. Your mother should not have to suffer. There are stronger drugs than morphine (like Fentanyl).

There has been a few here who deliberately witheld nutrition to spare their family. These decisions were carefully discussed in advance and mutually agreed on. None of them witheld hydration.