My mother died after a month in a coma, barring a few minutes one afternoon.
My father, a retired GP, said to my mother's consultant one afternoon, " I shall soon have to make a hard decision." The consultant replied kindly, "We are not there yet, but I am not interested in prolonging a patient's death throes unnecessarily. I treat the living, as long as there is some quality of life."
Written down this looks harsher than it was meant, as you cannot hear the respectful tone of voice it was said in, but I felt then, and still do, that the sentiment is worth considering and following.
When his time came, my father fought against pneumonia for three days, then gave up. His nurse agreed with my sister and I that the important thing was to ensure he was in no pain. He was 89, and for the three months preceeding his death, it had been plain to us all that he felt life had nothing more in store for him.
Obviously, your mother's dementia makes the situation even more distressing for you, and it is no help that like my mother she apparently never was willing to discuss the final phase of her life.
Talk frankly to the staff, listen to their evaluation and decide what you and they think is best.
As a total outsider, my feeling is that your mother is better off in surroundings she is accustomed to, even through the fog of dementia than in a hospital's pallitative or intensive dept.
It is very hard to watch our parents die, but the time leading up to their actual death is in my experience the worst part of the experience.
Come back if you need us.