"Is food a human right?"
I've answered this.
"What about dessert?"
Do we have life-saving surplus desserts lying around? Then save lives with it, yes. They have a right to life.
"What about sugar-free? What if someone thinks gluten is evil? Where do you draw the line?"
Do we have life-saving surplus foods that people aren't allergic to? Then starving people are entitled to it.
"What if someone is 90 years old, in good health, but they're diagnosed with a disease that will kill them within five years if it's not treated… Treatment will cost $1M."
Do we have a million dollars? We're the richest country in the world. Should be some spare cash around we were going to spend on landmines anyway. Cure the dying man. He has a right to medical care.
"Is it a "human right" that society pays for their treatment?"
Yes. The scope of human rights is not as narrow as you see it.
"How should society factor that case against kids with cancer, and limited health care budgets?"
Cure kids with cancer. Oh my God, how is this even a debate? Cure the f**king cancer children! Limited healthcare budgets: we're the richest country in the world. There are only so many children dying of cancer. This shouldn't be hard. If we can build giant statues of Jesus in Ohio, we have the spare cash to cure children with cancer.
"Worse, doesn't this create a moral hazard, since people would not be concerned about keeping themselves healthy if their healthcare is a "right" that society is responsible for providing?"
No. People still get an enormous personal benefit from keeping themselves healthy. Not going to the doctor at all is better than going to a free doctor.