Good try, but not too convincing I'm afraid. The same logic would justify leaving someone in a burning car on the basis that moving them might cause injury. The fact that the probability of survival if someone is not allowed any water is ZERO would imply that just about anything else would be better. As for being "peaceful", that is a lie. If someone's kidneys have failed, then denying them water might give them a "peaceful" death as their blood becomes so pulluted with waste products that they can no longer function (giving water in that case might dillute the waste products, but would also cause uncomfortable problems of its own). In someone whose kidneys are functioning, however, the kidneys will extract so much water from the blood that the person's skin, mucous membranes, etc. all dry up. Such a death is anything but peaceful. Although morphine might make it painless, the same could just as well be said for burning at the stake. And at least that doesn't put the victim through days of agony.
I think due to her lack of care and therapy that it's unlikely she improved on her own, but that's something we'll never know, and we could have.
Had Michael been convinced of his position, he surely wouldn't have minded silencing his critics by having her fail all the tests and then proceeding to remove the tube.