I hope your cousin (or her family) is consulting with hospice. They are experts in pain management and they can answer questions you are asking. I suspect that dying from an illness is pretty much the same regardless of the disease. My sympathies to you and yours.
God bless.
You're right, the idea of food "feeding" cancer is bizarre. Most people during the end stages lose their appetite though so don't try to force her to eat. We tend to feel better if we can nourish someone but near the end, people don't need/want food. They want pain relief and comfort, hearing the voices of the ones they love, feeling a gentle hand on theirs.
Not everyone vomits blood (thank goodness). I suppose if there is cancer in the stomach, it may happen.